PIONEER SOUTHWEST ENERGY PARTNERS L.P.

Exhibit 1.1

Execution Copy

PIONEER SOUTHWEST ENERGY PARTNERS L.P.

Common Units Representing Limited Partner Interests

Underwriting Agreement

November 11, 2009

Underwriting Agreement

November 11, 2009

UBS Securities LLC
Merrill Lynch, Pierce, Fenner & Smith Incorporated
Citigroup Global Markets Inc.
Wells Fargo Securities, LLC
   as Managing Underwriters
c/o UBS Securities LLC
299 Park Avenue
New York, New York 10171-0026

Ladies and Gentlemen:

          Pioneer Southwest Energy Partners L.P., a limited partnership organized under the laws of the State of Delaware (the “Partnership”), proposes to sell to the underwriters named in Schedule A annexed hereto (the “Underwriters”), for whom you are acting as representatives, an aggregate of 2,700,000 Common Units (the “Firm Units”), each representing a limited partner interest in the Partnership (the “Common Units”). In addition, solely for the purpose of covering over-allotments, the Partnership proposes to grant to the Underwriters the option to purchase from the Partnership up to an additional 405,000 Common Units (the “Additional Units”). The Firm Units and the Additional Units are hereinafter collectively sometimes referred to as the “Units.” The Units are described in the Prospectus which is referred to below.

          The Partnership has prepared and filed, in accordance with the provisions of the Securities Act of 1933, as amended, and the rules and regulations thereunder (collectively, the “Act”), with the Securities and Exchange Commission (the “Commission”) a registration statement on Form S-3 (File No. 333-162566) under the Act (the “registration statement”), including a prospectus, which registration statement incorporates by reference documents which the Partnership has filed, or will file, in accordance with the provisions of the Securities Exchange Act of 1934, as amended, and the rules and regulations thereunder (collectively, the “Exchange Act”). Such registration statement has become effective under the Act.

          Except where the context otherwise requires, “Registration Statement,” as used herein, means the registration statement, as amended at the time of such registration statement’s effectiveness for purposes of Section 11 of the Act, as such section applies to the respective Underwriters (the “Effective Time”), including (i) all documents filed as a part thereof or incorporated or deemed to be incorporated by reference therein, (ii) any information contained or incorporated by reference in a prospectus filed with the Commission pursuant to Rule 424(b) under the Act, to the extent such information is deemed, pursuant to Rule 430B or Rule 430C under the Act, to be part of the registration statement at the Effective Time, and (iii) any registration statement filed to register the offer and sale of Units pursuant to Rule 462(b) under the Act.

          The Partnership has furnished to you, for use by the Underwriters and by dealers in connection with the offering of the Units, copies of one or more preliminary prospectus

supplements, and the documents incorporated by reference therein, relating to the Units. Except where the context otherwise requires, “Pre-Pricing Prospectus,” as used herein, means each such preliminary prospectus supplement, in the form so furnished, including any basic prospectus (whether or not in preliminary form) furnished to you by the Partnership and attached to or used with such preliminary prospectus supplement. Except where the context otherwise requires, “Basic Prospectus,” as used herein, means any such basic prospectus and any basic prospectus furnished to you by the Partnership and attached to or used with the Prospectus Supplement (as defined below).

          Except where the context otherwise requires, “Prospectus Supplement,” as used herein, means the final prospectus supplement, relating to the Units, filed by the Partnership with the Commission pursuant to Rule 424(b) under the Act on or before the second business day after the date hereof (or such earlier time as may be required under the Act), in the form furnished by the Partnership to you for use by the Underwriters in connection with the offering of the Units.

          Except where the context otherwise requires, “Prospectus,” as used herein, means the Prospectus Supplement together with the Basic Prospectus attached to or used with the Prospectus Supplement.

          “Covered Free Writing Prospectuses,” as used herein, means (i) each “issuer free writing prospectus” (as defined in Rule 433(h)(1) under the Act), if any relating to the Units, which is not a Permitted Free Writing Prospectus and (ii) each Permitted Free Writing Prospectus.

          “Permitted Free Writing Prospectuses,” as used herein, means the documents listed on Schedule B attached hereto and each “road show” (as defined in Rule 433 under the Act), if any, related to the offering of the Units contemplated hereby that is a “written communication” (as defined in Rule 405 under the Act). The Underwriters have not offered or sold and will not offer or sell, without the Partnership’s consent, any Units by means of any “free writing prospectus” (as defined in Rule 405 under the Act) that is required to be filed by the Underwriters with the Commission pursuant to Rule 433 under the Act, other than a Permitted Free Writing Prospectus.

          “Disclosure Package,” as used herein, means any Pre-Pricing Prospectus or Basic Prospectus, in either case together with any combination of one or more of the Permitted Free Writing Prospectuses, if any.

          “Applicable Time,” as used herein, means the date and time that this Agreement is executed and delivered by the parties hereto.

          Any reference herein to the registration statement, the Registration Statement, any Basic Prospectus, any Pre-Pricing Prospectus, the Prospectus Supplement, the Prospectus or any Permitted Free Writing Prospectus shall be deemed to refer to and include the documents, if any, incorporated by reference, or deemed to be incorporated by reference, therein (the “Incorporated Documents”), including, unless the context otherwise requires, the documents, if any, filed as

exhibits to such Incorporated Documents. Any reference herein to the terms “amend,” “amendment” or “supplement” with respect to the Registration Statement, any Basic Prospectus, any Pre-Pricing Prospectus, the Prospectus Supplement, the Prospectus or any Permitted Free Writing Prospectus shall be deemed to refer to and include the filing of any document under the Exchange Act on or after the initial effective date of the Registration Statement, or the date of such Basic Prospectus, such Pre-Pricing Prospectus, the Prospectus Supplement, the Prospectus or such Permitted Free Writing Prospectus, as the case may be, and deemed to be incorporated therein by reference.

          As used in this Agreement, “business day” shall mean a day on which the New York Stock Exchange (the “NYSE”) is open for trading. The terms “herein,” “hereof,” “hereto,” “hereinafter” and similar terms, as used in this Agreement, shall in each case refer to this Agreement as a whole and not to any particular section, paragraph, sentence or other subdivision of this Agreement. The term “or,” as used herein, is not exclusive.

          The Partnership, Pioneer Natural Resources GP LLC, a Delaware limited liability company and the sole general partner of the Partnership (the “General Partner”), Pioneer Southwest Energy Partners USA LLC, a Delaware limited liability company (the “Operating Company”), PSE Finance Corporation, a Delaware corporation (“PSE Finance”), and all other Partnership subsidiaries are herein collectively referred to as the “Partnership Entities.”

          The Partnership and the Underwriters agree as follows:

     1. Sale and Purchase. Upon the basis of the representations and warranties and subject to the terms and conditions herein set forth, the Partnership agrees to sell to the respective Underwriters and each of the Underwriters, severally and not jointly, agrees to purchase from the Partnership the number of Firm Units set forth opposite the name of such Underwriter in Schedule A attached hereto, subject to adjustment in accordance with Section 8 hereof, at a purchase price of $19.82 per Unit. The Partnership is advised by you that the Underwriters intend (i) to make a public offering of their respective portions of the Firm Units as soon after the effectiveness of this Agreement as in your judgment is advisable and (ii) initially to offer the Firm Units upon the terms set forth in the Prospectus. You may from time to time increase or decrease the public offering price after the initial public offering to such extent as you may determine.

          In addition, the Partnership hereby grants to the several Underwriters the option (the “Over-Allotment Option”) to purchase, and upon the basis of the representations and warranties and subject to the terms and conditions herein set forth, the Underwriters shall have the right to purchase, severally and not jointly, from the Partnership, ratably in accordance with the number of Firm Units to be purchased by each of them, all or a portion of the Additional Units as may be necessary to cover over-allotments made in connection with the offering of the Firm Units, at the same purchase price per Unit to be paid by the Underwriters to the Partnership for the Firm Units. The Over-Allotment Option may be exercised by UBS Securities LLC (“UBS”) on behalf of the several Underwriters at any time and from time to time on or before the thirtieth day following the date of the Prospectus Supplement, by written notice to the Partnership. Such notice shall set forth the aggregate number of Additional Units as to which the

Over-Allotment Option is being exercised and the date and time when the Additional Units are to be delivered (any such date and time being herein referred to as an “additional time of purchase”); provided, however, that no additional time of purchase shall be earlier than the “time of purchase” (as defined below) nor earlier than the second business day after the date on which the Over-Allotment Option shall have been exercised nor later than the tenth business day after the date on which the Over-Allotment Option shall have been exercised. The number of Additional Units to be sold to each Underwriter shall be the number which bears the same proportion to the aggregate number of Additional Units being purchased as the number of Firm Units set forth opposite the name of such Underwriter on Schedule A hereto bears to the total number of Firm Units (subject, in each case, to such adjustment as UBS may determine to eliminate fractional Units), subject to adjustment in accordance with Section 8 hereof.

     2. Payment and Delivery. Payment of the purchase price for the Firm Units shall be made to the Partnership by Federal Funds wire transfer against delivery of the certificates for the Firm Units to you through the facilities of The Depository Trust Company (“DTC”) for the respective accounts of the Underwriters. Such payment and delivery shall be made at 9:00 A.M., New York City time, on November 16, 2009 (unless another time shall be agreed to by you and the Partnership or unless postponed in accordance with the provisions of Section 8 hereof). The time at which such payment and delivery are to be made is hereinafter sometimes called the “time of purchase.” Electronic transfer of the Firm Units shall be made to you at the time of purchase in such names and in such denominations as you shall specify.

          Payment of the purchase price for the Additional Units shall be made at the additional time of purchase in the same manner and at the same office and time of day as the payment for the Firm Units. Electronic transfer of the Additional Units shall be made to you at the additional time of purchase in such names and in such denominations as you shall specify.

          Deliveries of the documents described in Section 6 hereof with respect to the purchase of the Units shall be made at the offices of Baker Botts L.L.P. at 2001 Ross Avenue, Dallas, Texas 75201, at 9:00 A.M., New York City time, on the date of the closing of the purchase of the Firm Units or the Additional Units, as the case may be.

     3. Representations and Warranties of the Partnership. The Partnership represents and warrants to and agrees with each of the Underwriters that:

     (a) Registration. The Registration Statement has heretofore become effective under the Act or, with respect to any registration statement to be filed to register the offer and sale of Units pursuant to Rule 462(b) under the Act, will be filed with the Commission and become effective under the Act no later than 10:00 P.M., New York City time, on the date of determination of the public offering price for the Units; no stop order of the Commission preventing or suspending the use of any Basic Prospectus, any Pre-Pricing Prospectus, the Prospectus Supplement, the Prospectus or any Permitted Free Writing Prospectus, or the effectiveness of the Registration Statement, has been issued, and no proceedings for such purpose have been instituted or, to the Partnership’s knowledge, are contemplated by the Commission.

     (b) Form of Documents; No Material Misstatements or Omissions. The

Registration Statement complied when it became effective, complies as of the date hereof and, as amended or supplemented, at the time of purchase, and each additional time of purchase, if any, will comply, in all material respects, with the requirements of the Act; the conditions to the use of Form S-3 in connection with the offering and sale of the Units as contemplated hereby have been satisfied; the Registration Statement meets, and the offering and sale of the Units as contemplated hereby complies with, the requirements of Rule 415 under the Act (including, without limitation, Rule 415(a)(5) under the Act); the Registration Statement did not, as of the Effective Time, contain an untrue statement of a material fact or omit to state a material fact required to be stated therein or necessary to make the statements therein not misleading; each Pre-Pricing Prospectus complied, at the time it was filed with the Commission, and complies as of the date hereof, in all material respects with the requirements of the Act; at the Applicable Time, at the time of purchase and each additional time of purchase, if any, the Disclosure Package (together with the public offering price per Unit, number of Firm Units and number of Additional Units) does not include an untrue statement of a material fact or omit to state a material fact necessary in order to make the statements therein, in the light of the circumstances under which they were made, not misleading; each Basic Prospectus complied or will comply, as of its date and the date it was or will be filed with the Commission, complies as of the date hereof (if filed with the Commission on or prior to the date hereof) and, at the time of purchase and each additional time of purchase, if any, will comply, in all material respects, with the requirements of the Act; each of the Prospectus Supplement and the Prospectus will comply, as of the date that it is filed with the Commission, the date of the Prospectus Supplement, the time of purchase and each additional time of purchase, if any, in all material respects, with the requirements of the Act (in the case of the Prospectus, including, without limitation, Section 10(a) of the Act); on the date of the Prospectus Supplement and the date the Prospectus Supplement is filed with the Commission and at the time of purchase and each additional time of purchase, if any, any Prospectus Supplement or the Prospectus, as then amended or supplemented, will not include an untrue statement of a material fact or omit to state a material fact necessary in order to make the statements therein, in the light of the circumstances under which they were made, not misleading; each Permitted Free Writing Prospectus, when considered together with the Disclosure Package (together with the public offering price per Unit, number of Firm Units and number of Additional Units) as of the Applicable Time, does not include an untrue statement of a material fact or omit to state a material fact necessary in order to make the statements therein, in the light of the circumstances under which they were made, not misleading; provided, however, that the Partnership makes no representation or warranty in this Section 3(b) with respect to any statement contained in the Registration Statement, any Pre-Pricing Prospectus, the Prospectus or any Permitted Free Writing Prospectus in reliance upon and in conformity with information concerning an Underwriter and furnished in writing by or on behalf of such Underwriter through you to the Partnership expressly for use in the Registration Statement, such Pre-Pricing Prospectus, the Prospectus or such Permitted Free Writing Prospectus; each Incorporated Document, at the time such document was filed, or will be filed, with the Commission or at the time such document became or becomes effective, as applicable, complied or will comply, in all material respects, with the requirements of the Exchange Act and did not or will not, as applicable, include an untrue statement of a material fact or omit to state a

material fact necessary in order to make the statements therein, in the light of the circumstances under which they were made, not misleading.

     (c) Offering Material. Prior to the execution of this Agreement, the Partnership has not, directly or indirectly, offered or sold any Units by means of any “prospectus” (within the meaning of the Act) or used any “prospectus” (within the meaning of the Act) in connection with the offer or sale of the Units, in each case other than the Pre-Pricing Prospectuses and the Permitted Free Writing Prospectuses, if any; the Partnership has not, directly or indirectly, prepared, used or referred to any Permitted Free Writing Prospectus except in compliance with Rules 164 and 433 under the Act; the Partnership is not an “ineligible issuer” (as defined in Rule 405 under the Act) as of the eligibility determination date for purposes of Rules 164 and 433 under the Act with respect to the offering of the Units contemplated by the Registration Statement, without taking into account any determination by the Commission pursuant to Rule 405 under the Act that it is not necessary under the circumstances that the Partnership be considered an “ineligible issuer”; the parties hereto agree and understand that the content of any and all “road shows” (as defined in Rule 433 under the Act) related to the offering of the Units contemplated hereby is solely the property of the Partnership.

     (d) Formation and Qualification. Each of the Partnership Entities has been duly formed or incorporated and is validly existing as a limited partnership, limited liability company or corporation, as applicable, in good standing under the laws of its jurisdiction of organization with full power and authority to own or lease and to operate its properties currently owned or leased and conduct its business as currently conducted, in each case as described in the Registration Statement, the Pre-Pricing Prospectus, the Prospectus and the Permitted Free Writing Prospectuses, if any. Each of the Partnership Entities is duly qualified to do business as a foreign limited partnership, limited liability company or corporation, as applicable and is in good standing under the laws of each jurisdiction that requires such qualification, except where the failure to be so qualified could not have, individually or in the aggregate, a material adverse effect on the earnings, business, properties, assets, rights, operations, condition (financial or otherwise) or prospects of the Partnership Entities taken as a whole (a “Material Adverse Effect”), or subject the limited partners of the Partnership to any material liability or disability.

     (e) Power and Authority to Act as a General Partner. The General Partner has full power and authority to act as general partner of the Partnership in all material respects as described in the Registration Statement, the Pre-Pricing Prospectus, the Prospectus and the Permitted Free Writing Prospectuses, if any.

     (f) Ownership of Partnership Entities. All of the equity interests of each of the Partnership Entities are owned as set forth on Schedule C hereto. All of such equity interests are duly and validly authorized and issued in accordance with the general partnership, limited partnership or limited liability company agreements of each such Partnership Entity (the “Organizational Agreements”), are fully paid (to the extent required by the applicable Organizational Agreements) and nonassessable (except as such nonassessability may be affected, as applicable, by (i) Sections 17-607 and 17-804 of the Delaware Limited Partnership Act (the “Delaware LP Act”), or (ii) Sections 18-607 and

18-804 of the Delaware Limited Liability Company Act (the “Delaware LLC Act”). Such equity interests are owned as set forth on Schedule C free and clear of all liens, encumbrances, security interests, charges or other claims (“Liens”) (except restrictions on transferability as described in the Registration Statement, the Pre-Pricing Prospectus, the Prospectus and the Permitted Free Writing Prospectuses, if any).

     (g) Valid Issuance of the Units. The Units to be purchased by the Underwriters from the Partnership have been duly authorized for issuance and sale to the Underwriters pursuant to this Agreement and, when issued and delivered by the Partnership pursuant to this Agreement against payment of the consideration set forth herein, will be validly issued and fully paid to the extent required under the partnership agreement of the Partnership (the “Partnership Agreement”) and nonassessable (except as such nonassessability may be affected by matters described in Sections 17-607 and 17-804 of the Delaware LP Act).

     (h) Capitalization. As of the time of purchase, after giving effect to the offering of the Firm Units as contemplated by this Agreement, the issued and outstanding partnership interests of the Partnership will consist of 32,708,700 Common Units and a 0.1% general partner interest. All outstanding Common Units and the limited partner interest represented thereby have been duly authorized and validly issued in accordance with the Partnership Agreement and are fully paid to the extent required under the Partnership Agreement and nonassessable (except as such nonassessability may be affected by matters described in Sections 17-607 and 17-804 of the Delaware LP Act).

     (i) No Other Subsidiaries. Except as described in the Registration Statement, the Pre-Pricing Prospectus, the Prospectus and the Permitted Free Writing Prospectuses, if any, none of the Partnership Entities own, directly or indirectly, any equity or long-term debt securities of any corporation, partnership, limited liability company, joint venture, association or other entity.

     (j) No Preemptive Rights, Registration Rights or Options. Except as identified in the Registration Statement, the Pre-Pricing Prospectus and the Prospectus, there are no (i) preemptive rights or other rights to subscribe for or to purchase, nor any restriction upon the voting or transfer of, any equity securities of the Partnership Entities or (ii) outstanding options or warrants to purchase any securities of the Partnership Entities. Except for such rights that have been waived or as described in the Registration Statement, the Pre-Pricing Prospectus and the Prospectus, neither the filing of the Registration Statement nor the offering or sale of the Units as contemplated by this Agreement gives rise to any rights for or relating to the registration of any Units or other securities of the Partnership.

     (k) Authority and Authorization. The Partnership has all requisite power and authority to execute and deliver this Agreement and perform its respective obligations hereunder. The Partnership has all requisite partnership power and authority to issue, sell and deliver the Units, in accordance with and upon the terms and conditions set forth in this Agreement, the Partnership Agreement, the Registration Statement, the Pre-Pricing Prospectus and the Prospectus. All corporate, partnership and limited liability company

action, as the case may be, required to be taken by the Partnership, the General Partner, or any of their members or partners, for the authorization, issuance, sale and delivery of the Units and the consummation of the transactions contemplated by this Agreement, shall have been validly taken.

     (l) Authorization of this Agreement. This Agreement has been duly authorized, executed and delivered by the Partnership.

     (m) Enforceability of Organizational Agreements. The Organizational Agreements have been duly authorized, executed and delivered by the parties thereto and are valid and legally binding agreements of such parties, enforceable against such parties in accordance with their respective terms; provided that, with respect to each agreement described in this Section 1(m), the enforceability thereof may be limited by bankruptcy, insolvency, fraudulent transfer, reorganization, moratorium and similar laws relating to or affecting creditors’ rights generally and by general principles of equity (regardless of whether such enforceability is considered in a proceeding in equity or at law); and provided further that, the indemnity, contribution and exoneration provisions contained in any of such agreements may be limited by applicable laws and public policy.

     (n) No Conflicts. None of (i) the offering, issuance or sale by the Partnership of the Units or (ii) the execution, delivery and performance of this Agreement, (A) conflicts or will conflict with or constitutes or will constitute a violation of the Organizational Agreements or the certificate of formation or conversion, certificate or articles of incorporation, bylaws or other constituent document (collectively, the “Organizational Documents”) of any of the Partnership Entities, (B) conflicts or will conflict with or constitutes or will constitute a breach or violation of, or a default (or an event that, with notice or lapse of time or both, would constitute such a default) under any indenture, mortgage, deed of trust, loan agreement, lease or other agreement or instrument to which any of the Partnership Entities is a party or by which any of them or any of their respective properties may be bound, (C) violates or will violate any statute, law or regulation or any order, judgment, decree or injunction of any court or governmental agency or body directed to any of the Partnership Entities or any of their properties in a proceeding to which any of them or their property is a party or (D) results or will result in the creation or imposition of any Lien upon any property or assets of any of the Partnership Entities, which conflicts, breaches, violations, defaults or Liens, in the case of clauses (B), (C) or (D), would reasonably be expected to have a Material Adverse Effect or materially impair the ability of the Partnership to consummate the transactions provided for in this Agreement.

     (o) No Consents. No permit, consent, approval, authorization, order, registration, filing or qualification of or with any court, governmental agency or body having jurisdiction over any of the Partnership Entities or any of their properties or assets is required in connection with the offering, issuance or sale by the Partnership of the Units, the execution, delivery and performance of this Agreement by the Partnership Entities, or the consummation of the transactions contemplated by this Agreement except for such permits, consents, approvals and similar authorizations required under the Act, the Exchange Act and blue sky laws of any jurisdiction.

     (p) No Defaults. None of the Partnership Entities is in (i) violation of its Organizational Documents, or of any statute, law, rule or regulation, or any judgment, order, injunction or decree of any court, governmental agency or body or arbitrator having jurisdiction over any of the Partnership Entities or any of their properties or assets or (ii) breach, default (or an event which, with notice or lapse of time or both, would constitute such an event) or violation in the performance of any obligation, agreement or condition contained in any indenture, mortgage, deed of trust, loan agreement, lease or other agreement or instrument to which it is a party or by which it or any of its properties may be bound, which in the case of either (i) or (ii) would reasonably be expected to have, if continued, a Material Adverse Effect or materially impair the ability of the Partnership to consummate the transactions provided for in this Agreement.

     (q) Conformity of Units to Description. The Units, when issued and delivered in accordance with the terms of the Partnership Agreement and this Agreement against payment therefor as provided therein and herein, will conform in all material respects to the description thereof contained in the Registration Statement, the Pre-Pricing Prospectus, the Prospectus and any Permitted Free Writing Prospectus.

     (r) No Labor Dispute. No labor problem or dispute with the Partnership Entities’ employees exists or is threatened or imminent that would reasonably be expected to have a Material Adverse Effect, except as set forth in or contemplated in the Registration Statement, the Pre-Pricing Prospectus, the Prospectus and any Permitted Free Writing Prospectus.

     (s) Financial Statements. The historical financial statements and schedules filed as part of the Registration Statement or included or incorporated by reference in the Pre-Pricing Prospectus, the Prospectus or any Permitted Free Writing Prospectus (and any amendment or supplement thereto) present fairly the financial condition, results of operations and cash flows of the entities purported to be shown thereby on the basis shown therein as of the dates and for the periods indicated, comply as to form with the applicable accounting requirements of the Act and have been prepared in conformity with generally accepted accounting principles applied on a consistent basis throughout the periods involved (except as otherwise noted therein). The selected historical financial information incorporated by reference in the Registration Statement, the Pre-Pricing Prospectus, the Prospectus or any Permitted Free Writing Prospectus (and any amendment or supplement thereto) is accurately presented in all material respects and prepared on a basis consistent with the audited and unaudited historical financial statements and pro forma financial statements, as applicable, from which it has been derived. The pro forma financial statements incorporated by reference in the Registration Statement, the Pre-Pricing Prospectus, the Prospectus or any Permitted Free Writing Prospectus (and any amendment or supplement thereto) include assumptions that provide a reasonable basis for presenting the significant effects directly attributable to the transactions and events described therein, the related pro forma adjustments give appropriate effect to those assumptions, and the pro forma adjustments reflect the proper application of those adjustments to the historical financial statement amounts in the pro forma financial statements incorporated by reference in Registration Statement, the Pre-Pricing Prospectus, the Prospectus or any Permitted Free Writing Prospectus (and any

amendment or supplement thereto). The pro forma financial statements incorporated by reference in the Registration Statement, the Pre-Pricing Prospectus, the Prospectus or any Permitted Free Writing Prospectus (and any amendment or supplement thereto) comply as to form in all material respects with the applicable accounting requirements of Regulation S-X under the Act and the pro forma adjustments have been properly applied to the historical amounts in the compilation of those statements.

     (t) Independent Public Accountants. Ernst & Young LLP, who has audited the audited financial statements and schedules included in the Registration Statement, the Pre-Pricing Prospectus and the Prospectus (or any amendment or supplement thereto) and delivered its reports with respect to the audited financial statements and schedules included in the Registration Statement, the Pre-Pricing Prospectus and the Prospectus (or any amendment or supplement thereto), is an independent registered public accounting firm with respect to the Partnership within the meaning of the Act and the rules and regulations of the Public Company Accounting Oversight Board (United States).

     (u) Litigation. Except as described in the Registration Statement, the Pre-Pricing Prospectus, the Prospectus or any Permitted Free Writing Prospectus, there is (i) no action, suit or proceeding before or by any court, arbitrator or governmental agency, body or official, domestic or foreign, now pending or, to the knowledge of the Partnership, threatened, to which any of the Partnership Entities is or may be a party or to which the business or property of any of the Partnership Entities is or may be subject, (ii) no statute, rule, regulation or order that has been enacted, adopted or issued by any governmental agency and (iii) no injunction, restraining order or order of any nature issued by a federal or state court or foreign court of competent jurisdiction to which any of the Partnership Entities is or may be subject, that, in the case of clauses (i), (ii) and (iii) above, is reasonably expected to (A) have a Material Adverse Effect, (B) prevent or result in the suspension of the offering and issuance of the Units or (C) in any manner draw into question the validity of this Agreement.

     (v) Title to Partnership Assets. The Partnership Entities have good and indefeasible title to all real property and good title to all personal property described in the Registration Statement, the Pre-Pricing Prospectus, the Prospectus or any Permitted Free Writing Prospectus as owned by the Partnership Entities (the “Partnership Assets”), free and clear of all Liens, except (i) as described, and subject to limitations contained, in the Registration Statement, the Pre-Pricing Prospectus, the Prospectus or any Permitted Free Writing Prospectus; (ii) that arise under the Partnership’s credit agreement with Bank of America, N.A., dated as of October 29, 2007, as amended (the “Credit Agreement”); or (iii) such as do not materially interfere with the use of such properties taken as a whole as they have been used in the past and are proposed to be used in the future as described in the Registration Statement, the Pre-Pricing Prospectus, the Prospectus or any Permitted Free Writing Prospectus; provided that, with respect to any real property and buildings held under lease by the Partnership Entities, such real property and buildings are held under valid and subsisting and enforceable leases with such exceptions as do not materially interfere with the use of the properties of the Partnership Entities taken as a whole as they have been used in the past as described in the Registration Statement, the Pre-Pricing Prospectus, the Prospectus and any Permitted

Free Writing Prospectus and are proposed to be used in the future as described in the Registration Statement, the Pre-Pricing Prospectus, the Prospectus and any Permitted Free Writing Prospectus.

     (w) Rights-of-Way. The Partnership Entities have such easements or rights-of-way from each person (collectively, “rights-of-way”) as are necessary to conduct their business in the manner described, and subject to the limitations contained, in the Registration Statement, the Pre-Pricing Prospectus, the Prospectus or any Permitted Free Writing Prospectus, except for (i) qualifications, reservations and encumbrances that would not have a Material Adverse Effect and (ii) such rights-of-way that, if not obtained, would not have a Material Adverse Effect; other than as set forth, and subject to the limitations contained, in the Registration Statement, the Pre-Pricing Prospectus, the Prospectus or any Permitted Free Writing Prospectus, the Partnership Entities have fulfilled and performed all their material obligations with respect to such rights-of-way and no event has occurred that allows, or after notice or lapse of time would allow, revocation or termination thereof or would result in any impairment of the rights of the holder of any such rights-of-way, except for such revocations, terminations and impairments that would not have a Material Adverse Effect and, except as described in the Registration Statement, the Pre-Pricing Prospectus, the Prospectus or any Permitted Free Writing Prospectus, none of such rights-of-way contains any restriction that is materially burdensome to the Partnership Entities, taken as a whole.

     (x) Transfer Taxes. There are no transfer taxes or other similar fees or charges under federal law or the laws of any state, or any political subdivision thereof, required to be paid in connection with the execution and delivery of this Agreement or the issuance by the Partnership or sale by the Partnership of the Units.

     (y) Tax Returns. Each of the Partnership Entities has filed all foreign, federal, state and local tax returns that are required to be filed or has requested extensions thereof, except in any case in which the failure so to file would not have a Material Adverse Effect. Except as set forth in or contemplated in the Registration Statement, the Pre-Pricing Prospectus, the Prospectus or any Permitted Free Writing Prospectus, each of the Partnership Entities has paid all taxes required to be paid by it and any other assessment, fine or penalty levied against it, to the extent that any of the foregoing is due and payable, except for any such assessment, fine or penalty that is currently being contested in good faith or as would not have a Material Adverse Effect.

     (z) Insurance. The Partnership Entities carry or are entitled to the benefits of insurance relating to the Partnership Assets in such amounts and covering such risks as is commercially reasonable, and all such insurance is in full force and effect. None of the Partnership Entities has any reason to believe that they will not be able (i) to renew their existing insurance coverage relating to the Partnership Assets as and when such policies expire or (ii) to obtain comparable coverage relating to the Partnership Assets from similar institutions as may be necessary or appropriate to conduct such business as now conducted and at a cost that would not reasonably be expected to have a Material Adverse Effect.

     (aa) Distribution Restrictions. No subsidiary of the Partnership is currently prohibited, directly or indirectly, from paying any distributions to the Partnership, from making any other distribution on such subsidiary’s equity interests, from repaying to the Partnership any loans or advances to such subsidiary from the Partnership or from transferring any of such subsidiary’s property or assets to the Partnership or any other subsidiary of the Partnership, except as described in or contemplated by the Registration Statement, the Pre-Pricing Prospectus, the Prospectus or any Permitted Free Writing Prospectus or arising under the Credit Agreement.

     (bb) Possession of Licenses and Permits. The Partnership Entities possess such permits, licenses, approvals, consents and other authorizations (collectively, “Governmental Licenses”) issued by the appropriate federal, state, local or foreign regulatory agencies or bodies necessary to conduct the business associated with the Partnership Assets, except where the failure so to possess would not reasonably be expected to result in a Material Adverse Effect; the Partnership Entities are in compliance with the terms and conditions of all such Governmental Licenses, except where the failure so to comply would not reasonably be expected to result in a Material Adverse Effect. All of the Governmental Licenses are valid and in full force and effect, except when the invalidity of such Governmental Licenses or the failure of such Governmental Licenses to be in full force and effect would not reasonably be expected to result in a Material Adverse Effect. The Partnership Entities have not received any notice of proceedings relating to the revocation or modification of any such Governmental Licenses that, if the subject of an unfavorable decision, ruling or finding, would reasonably be expected to result in a Material Adverse Effect.

     (cc) Environmental Laws. With respect to the Partnership Assets, each of the Partnership Entities (i) is in compliance with all applicable federal, state and local laws and regulations relating to the prevention of pollution or protection of the environment or imposing liability or standards of conduct concerning any Hazardous Material (as defined below) (“Environmental Laws”), (ii) has received all permits required of them under applicable Environmental Laws to conduct their respective businesses as presently conducted, (iii) is in compliance with all terms and conditions of any such permits and (iv) does not have any liability in connection with the release into the environment of any Hazardous Material, except where such noncompliance with Environmental Laws, failure to receive required permits, failure to comply with the terms and conditions of such permits or liability in connection with such releases would not have a Material Adverse Effect. The term “Hazardous Material” means (A) any “hazardous substance” as defined in the Comprehensive Environmental Response, Compensation and Liability Act of 1980, as amended, (B) any “hazardous waste” as defined in the Resource Conservation and Recovery Act, as amended, (C) any petroleum or petroleum product, (D) any polychlorinated biphenyl and (E) any pollutant or contaminant or hazardous, dangerous or toxic chemical, material, waste or substance regulated under or within the meaning of any applicable Environmental Law. In the ordinary course of business, the Partnership Entities periodically review the effect of Environmental Laws on their business, operations and properties, in the course of which they identify and evaluate costs and liabilities that are reasonably likely to be incurred pursuant to such Environmental Laws (including, without limitation, any capital or operating expenditures required for clean-

up, closure of properties or compliance with Environmental Laws, or any permit, license or approval, any related constraints on operating activities and any potential liabilities to third parties). On the basis of such review, the Partnership Entities have reasonably concluded that such associated costs and liabilities relating to the Partnership Assets would not have a Material Adverse Effect.

     (dd) Certain Relationships and Related Transactions. No relationship, direct or indirect, exists between or among any Partnership Entity, on the one hand, and the directors, officers, stockholders, affiliates, customers or suppliers of any Partnership Entity, on the other hand, that is required to be described in the Registration Statement, Pre-Pricing Prospectus, the Prospectus and any Permitted Free Writing Prospectus and is not so described.

     (ee) ERISA. Each Partnership Entity is in compliance in all material respects with all presently applicable provisions of the Employee Retirement Income Security Act of 1974, as amended, including the regulations and published interpretations thereunder (“ERISA”). No “reportable event” (as defined in ERISA) has occurred with respect to any “pension plan” (as defined in ERISA) for which any Partnership Entity (after giving effect to the Transactions) would have any liability, excluding any reportable event for which a waiver could apply; no Partnership Entity expects to incur liability under (i) Title IV of ERISA with respect to termination of, or withdrawal from, any “pension plan” or (ii) Sections 412 or 4971 of the Internal Revenue Code of 1986, as amended, including the regulations and published interpretations thereunder (the “Code”). Each “pension plan” for which any Partnership Entity would have any liability that is intended to be qualified under Section 401(a) of the Code has been determined by the Internal Revenue Service to be so qualified and nothing has occurred, whether by action or by failure to act, that could reasonably be expected to cause the loss of such qualification.

     (ff) Description of Legal Proceedings and Contracts; Filing of Exhibits. There are no legal or governmental proceedings pending or, to the knowledge of the Partnership, threatened or contemplated, against any of the Partnership Entities, or to which any of the Partnership Entities is a party, or to which any of their properties or assets is subject, that are required to be described in the Registration Statement, Pre-Pricing Prospectus, the Prospectus and any Permitted Free Writing Prospectus that are not described as required, and there are no agreements, contracts, indentures, leases or other instruments that are required to be described in the Registration Statement, Pre-Pricing Prospectus, the Prospectus and any Permitted Free Writing Prospectus or to be filed as an exhibit to the Registration Statement that are not described or filed as required by the Act or the Exchange Act. The statements included in the Registration Statement, Pre-Pricing Prospectus, the Prospectus and any Permitted Free Writing Prospectus, insofar as such statements summarize legal matters, agreements, documents or proceedings discussed therein, are accurate summaries of such legal matters, agreements, documents or proceedings.

     (gg) Sarbanes-Oxley Act of 2002. The Partnership is in compliance in all material respects with all applicable provisions of the Sarbanes-Oxley Act of 2002, the rules and regulations promulgated in connection therewith and the rules of the NYSE that

are effective and applicable to the Partnership.

     (hh) Investment Company. None of the Partnership Entities is nor, after giving effect to the offering and sale of the Units and the application of the proceeds thereof as described in the Pre-Pricing Prospectus and the Prospectus under the caption “Use of Proceeds”, will any of the Partnership Entities be, an “investment company” or a company “controlled by” an “investment company,” each as defined in the Investment Company Act of 1940, as amended (the “Investment Company Act”) and the rules and regulations thereunder.

     (ii) Books and Records. Each Partnership Entity maintains a system of internal accounting controls sufficient to provide reasonable assurance that (i) transactions are executed in accordance with management’s general or specific authorizations; (ii) transactions are recorded as necessary to permit preparation of financial statements in conformity with generally accepted accounting principles and to maintain asset accountability; (iii) access to assets is permitted only in accordance with management’s general or specific authorizations; and (iv) the recorded accountability for assets is compared with the existing assets at reasonable intervals and appropriate action is taken with respect to any differences. Each Partnership Entity’s internal controls over financial reporting are effective and none of the Partnership Entities is aware of any material weakness in their internal control over financial reporting.

     (jj) Disclosure Controls and Procedures. (i) Each Partnership Entity has established and maintains “disclosure controls and procedures” (to the extent required by and as such term is defined in Rule 13a-15 under the Exchange Act), (ii) such disclosure controls and procedures are designed to ensure that all information required to be disclosed by the Partnership in the reports it files or will file or submit under the Exchange Act, as applicable, is accumulated and communicated to management of the General Partner, including its respective principal executive officers and principal financial officers, as appropriate, to allow timely decisions regarding required disclosure to be made, (iii) such disclosure controls and procedures have been evaluated for effectiveness as of the period covered by the Partnership’s most recent period report filed with the Commission and (iv) such disclosure controls and procedures are effective in all material respects to perform the functions for which they were established to the extent required by Rule 13a-15 of the Exchange Act.

     (kk) Market Stabilization. None of the Partnership Entities has taken, directly or indirectly, any action designed to or that would constitute or that might reasonably be expected to cause or result in, under the Exchange Act or otherwise, stabilization or manipulation of the price of any security of the Partnership to facilitate the sale or resale of the Units.

     (ll) Foreign Corrupt Practices Act. No Partnership Entity nor, to the knowledge of the Partnership, any director, officer, agent, employee or affiliate of any Partnership Entity is aware of or has taken any action, directly or indirectly, that would result in a violation by such Partnership Entity of the Foreign Corrupt Practices Act of 1977, as amended, and the rules and regulations thereunder (collectively, the “FCPA”),

including, without limitation, making use of the mails or any means or instrumentality of interstate commerce corruptly in furtherance of an offer, payment, promise to pay or authorization of the payment of any money, or other property, gift, promise to give, or authorization of the giving of anything of value to any “foreign official” (as such term is defined in the FCPA) or any foreign political party or official thereof or any candidate for foreign political office, in contravention of the FCPA and the Partnership Entities and, to the knowledge of the Partnership, their affiliates have conducted the businesses of the Partnership Entities in compliance with the FCPA and have instituted and maintain policies and procedures designed to ensure, and which are reasonably expected to continue to ensure, continued compliance therewith by the Partnership Entities.

     (mm) Money Laundering Laws. The operations of the Partnership Entities are and have been conducted at all times in compliance with applicable financial recordkeeping and reporting requirements of the Currency and Foreign Transactions Reporting Act of 1970, as amended, the money laundering statutes of all jurisdictions, the rules and regulations thereunder and any related or similar rules, regulations or guidelines, issued, administered or enforced by any governmental agency (collectively, the “Money Laundering Laws”) and no action, suit or proceeding by or before any court or governmental agency, authority or body or any arbitrator involving any of the Partnership Entities with respect to the Money Laundering Laws is pending or, to the best knowledge of the Partnership, threatened.

     (nn) Office of Foreign Assets Control. No Partnership Entity nor, to the knowledge of the Partnership, any director, officer, agent or employee of any Partnership Entity or any of its subsidiaries is currently subject to any U.S. sanctions administered by the Office of Foreign Assets Control of the U.S. Treasury Department (“OFAC”); and the Partnership will not directly or indirectly use the proceeds of the offering, or lend, contribute or otherwise make available such proceeds to any subsidiary, joint venture partner or other person or entity, for the purpose of financing the activities of any person currently subject to any U.S. sanctions administered by OFAC.

     (oo) Statistical Data. All statistical and market-related data included in the Registration Statement, Pre-Pricing Prospectus, the Prospectus and any Permitted Free Writing Prospectus is based on or derived from sources that the Partnership believes to be reliable and accurate, and the Partnership has obtained the written consent to the use of such data from such sources to the extent required.

     (pp) Independent Petroleum Engineers. Netherland, Sewell & Associates, Inc. (“NSAI”), whose reports are referenced in the Pre-Pricing Prospectus and the Prospectus, and who has delivered the letter referenced to in Section 6(i) hereof, was, as of the date of such reports, and is, as of the date hereof, an independent engineering firm with respect to the Partnership.

     (qq) Information Underlying Reserve Report. The factual information underlying the estimates of proved oil and gas reserves of the Partnership Entities, which was supplied by the Partnership Entities to NSAI for the purposes of auditing the Partnership’s internally prepared reserve report and preparing the letter (the “Reserve

Report Letter”) of NSAI, including, without limitation, production volumes, costs of operation and development, current prices for production, agreements relating to current and future operations and sales of production, was true and correct in all material respects on the dates such estimates were made and such information was supplied and was prepared in accordance with customary industry practices; other than normal production of the reserves and intervening market commodity price fluctuations, the Partnership Entities are not aware of any facts or circumstances that would result in a material adverse change in the reserves, or the present value of future net cash flows therefrom, as described in the Registration Statement, Pre-Pricing Prospectus, the Prospectus and any Permitted Free Writing Prospectus and as reflected in the Reserve Report Letter; estimates of such reserves and present values as described in the Registration Statement, Pre-Pricing Prospectus, the Prospectus and any Permitted Free Writing Prospectus and reflected in the Reserve Report Letter comply in all material respects with the applicable requirements of Regulation S-X and Industry Guide 2 under the Act.

     (rr) No Material Adverse Change. Subsequent to the respective dates as of which information is given in the Registration Statement, the Pre-Pricing Prospectus, the Prospectus and any Permitted Free Writing Prospectus, in each case excluding any amendments or supplements to the foregoing made after the execution of this Agreement, there has not been any material adverse change, or any development involving a prospective material adverse change, in the business, properties, management, financial condition or results of operations of the Partnership Entities taken as a whole.

          Any certificate signed by any officer of the General Partner and delivered to the Underwriters or counsel for the Underwriters in connection with the offering of the Units shall be deemed a representation and warranty by such entity, as to matters covered thereby, to each Underwriter.

4. Certain Covenants of the Partnership. The Partnership agrees:

     (a) to furnish such information as may be required and otherwise to cooperate in qualifying the Units for offering and sale under the securities or blue sky laws of such states or other jurisdictions as you may designate and to maintain such qualifications in effect so long as you may request for the distribution of the Units; provided, however, that the Partnership shall not be required to qualify as a foreign corporation or to consent to the service of process under the laws of any such jurisdiction (except service of process with respect to the offering and sale of the Units); and to promptly advise you of the receipt by the Partnership Entities of any notification with respect to the suspension of the qualification of the Units for offer or sale in any jurisdiction or the initiation or threatening of any proceeding for such purpose;

     (b) to make available to the Underwriters in New York City, as soon as practicable after this Agreement becomes effective, and thereafter from time to time to furnish to the Underwriters, as many copies of the Prospectus (or of the Prospectus as amended or supplemented if the Partnership shall have made any amendments or supplements thereto after the effective date of the Registration Statement) as the Underwriters may request for the purposes contemplated by the Act; in case any

Underwriter is required to deliver (whether physically or through compliance with Rule 172 under the Act or any similar rule), in connection with the sale of the Units, a prospectus after the nine-month period referred to in Section 10(a)(3) of the Act, or after the time a post-effective amendment to the Registration Statement is required pursuant to Item 512(a) of Regulation S-K under the Act, the Partnership will prepare, at its expense, promptly upon request such amendment or amendments to the Registration Statement and the Prospectus as may be necessary to permit compliance with the requirements of Section 10(a)(3) of the Act or Item 512(a) of Regulation S-K under the Act, as the case may be;

     (c) if, at the time this Agreement is executed and delivered, it is necessary or appropriate for a post-effective amendment to the Registration Statement, or a Registration Statement under Rule 462(b) under the Act, to be filed with the Commission and become effective before the Units may be sold, to use its reasonable best efforts to cause such post-effective amendment or such Registration Statement to be filed and become effective, and pay any applicable fees in accordance with the Act, as soon as possible; and advise you promptly and, if requested by you, confirm such advice in writing, (i) when such post-effective amendment or such Registration Statement has become effective, and (ii) if Rule 430A under the Act is used, when the Prospectus is filed with the Commission pursuant to Rule 424(b) under the Act (which the Partnership agrees to file in a timely manner in accordance with such Rules);

     (d) if, at any time during the period when a prospectus is required by the Act to be delivered (whether physically or through compliance with Rule 172 under the Act or any similar rule) in connection with any sale of Units, the Registration Statement shall cease to comply with the requirements of the Act with respect to eligibility for the use of the form on which the Registration Statement was filed with the Commission, to (i) promptly notify you, (ii) promptly file with the Commission a new registration statement under the Act, relating to the Units, or a post-effective amendment to the Registration Statement, which new registration statement or post-effective amendment shall comply with the requirements of the Act and shall be in a form satisfactory to you, (iii) use its reasonable best efforts to cause such new registration statement or post-effective amendment to become effective under the Act as soon as practicable, (iv) promptly notify you of such effectiveness and (v) take all other action necessary or appropriate to permit the public offering and sale of the Units to continue as contemplated in the Prospectus; all references herein to the Registration Statement shall be deemed to include each such new registration statement or post-effective amendment, if any;

     (e) to advise you promptly, confirming such advice in writing, of any request by the Commission for amendments or supplements to the Registration Statement, any Pre-Pricing Prospectus, the Prospectus or any Permitted Free Writing Prospectus or for additional information with respect thereto, or of notice of institution of proceedings for, or the entry of a stop order, suspending the effectiveness of the Registration Statement and, if the Commission should enter a stop order suspending the effectiveness of the Registration Statement, to use its reasonable best efforts to obtain the lifting or removal of such order as soon as possible; to advise you promptly of any proposal to amend or

supplement the Registration Statement, any Pre-Pricing Prospectus or the Prospectus, and to provide you and Underwriters’ counsel copies of any such documents for review and comment a reasonable amount of time prior to any proposed filing and to file no such amendment or supplement to which you shall reasonably object in writing;

     (f) subject to Section 4(e) hereof, to file promptly all reports and documents and any preliminary or definitive proxy or information statement required to be filed by the Partnership with the Commission in order to comply with the Exchange Act for so long as a prospectus is required by the Act to be delivered (whether physically or through compliance with Rule 172 under the Act or any similar rule) in connection with any sale of Units;

     (g) to advise the Underwriters promptly of the happening of any event within the period during which a prospectus is required by the Act to be delivered (whether physically or through compliance with Rule 172 under the Act or any similar rule) in connection with any sale of Units, which event could require the making of any change in the Prospectus then being used so that the Prospectus would not include an untrue statement of a material fact or omit to state a material fact necessary in order to make the statements therein, in the light of the circumstances under which they are made, not misleading, and to advise the Underwriters promptly if, during such period, it shall become necessary to amend or supplement the Prospectus to cause the Prospectus to comply with the requirements of the Act, and, in each case, during such time, subject to Section 4(e) hereof, to prepare and furnish, at the Partnership’s expense, to the Underwriters promptly such amendments or supplements to such Prospectus as may be necessary to reflect any such change or to effect such compliance;

     (h) to make generally available to its security holders, and to deliver to you, an earnings statement of the Partnership which will satisfy, on a timely basis, the provisions of Section 11(a) of the Act and Rule 158 under the Act;

     (i) to furnish to you seven copies of the Registration Statement, as initially filed with the Commission, and of all amendments thereto (including all exhibits thereto and documents incorporated by reference therein) and sufficient copies of the foregoing (other than exhibits) for distribution of a copy to each of the other Underwriters;

     (j) to apply the net proceeds from the sale of the Units in the manner set forth under the caption “Use of Proceeds” in the Prospectus Supplement;

     (k) to pay all costs, expenses, fees and taxes in connection with (i) the preparation and filing of the Registration Statement, each Basic Prospectus, each Pre-Pricing Prospectus, the Prospectus Supplement, the Prospectus, each Permitted Free Writing Prospectus and any amendments or supplements thereto, and the printing and furnishing of copies of each thereof to the Underwriters and to dealers (including costs of mailing and shipment), (ii) the registration, issue, sale and delivery of the Units including any stock or transfer taxes and stamp or similar duties payable upon the sale, issuance or delivery of the Units to the Underwriters, (iii) the printing and delivering to the

Underwriters of this Agreement, (iv) the qualification of the Units for offering and sale under state or foreign laws and the determination of their eligibility for investment under state or foreign law (including the legal fees and filing fees and other disbursements of counsel for the Underwriters) and the printing and furnishing of copies of any blue sky surveys or legal investment surveys to the Underwriters and to dealers, (v) any listing of the Units on any securities exchange or qualification of the Units for quotation on the NYSE and any registration thereof under the Exchange Act, (vi) any filing for review of the public offering of the Units by the Financial Industry Regulatory Authority (“FINRA”), including the reasonable legal fees and filing fees and other disbursements of counsel to the Underwriters relating to FINRA matters, (vii) the fees and disbursements of any transfer agent or registrar for the Units, (viii) the costs and expenses of the Partnership relating to presentations or meetings undertaken in connection with the marketing of the offering and sale of the Units to prospective investors and the Underwriters’ sales forces, including, without limitation, expenses associated with the production of road show slides and graphics, fees and expenses of any consultants engaged in connection with the road show presentations, travel, lodging and other expenses incurred by the officers of the General Partner and any such consultants, and the cost of any aircraft chartered in connection with the road show, and (ix) the performance of the Partnership’s other obligations hereunder;

     (l) to comply with Rule 433(d) under the Act (without reliance on Rule 164(b) under the Act) and with Rule 433(g) under the Act;

     (m) without the prior written consent of the Managing Underwriters, offer, sell, contract to sell, pledge, or otherwise dispose of (or enter into any transaction which is designed to, or might reasonably be expected to, result in the disposition (whether by actual disposition or effective economic disposition due to cash settlement or otherwise) by the Partnership or any affiliate of the Partnership or any person in privity with the Partnership or any affiliate of the Partnership), directly or indirectly, including the filing (or participation in the filing) of a registration statement with the Commission in respect of, or establish or increase a put equivalent position or liquidate or decrease a call equivalent position within the meaning of Section 16 of the Exchange Act, any other Common Units or any securities convertible into, or exercisable, or exchangeable for, Common Units; or publicly announce an intention to effect any such transaction, for a period of 60 days after the date of this Agreement, provided, however, that the Partnership may (i) issue and sell the Units pursuant to this Agreement, (ii) issue and sell Common Units pursuant to any employee benefit plan of the Partnership or the General Partner described in the Registration Statement (excluding the exhibits thereto), each Pre-Pricing Prospectus and the Prospectus, (iii) issue Common Units issuable upon the conversion of securities or the exercise of warrants outstanding at the Execution Time, and (iv) issue Common Units as consideration for any merger and acquisition made by the Partnership; provided, however, that in the case of any issuance pursuant to this clause (iv), in the case of any merger or acquisition other than the acquisition of all of the outstanding equity of a publicly traded entity, each recipient of any such Common Units shall sign and deliver an agreement in substantially the form of Exhibit A hereto;

     (n) not, at any time at or after the execution of this Agreement, to, directly or indirectly, offer or sell any Units by means of any “prospectus” (within the meaning of the Act), or use any “prospectus” (within the meaning of the Act) in connection with the offer or sale of the Units, in each case other than the Prospectus or pursuant to any employee benefit plan of the Partnership or the General Partner described in the Registration Statement (excluding the exhibits thereto);

     (o) not to, and to cause the Partnership Entities not to, take, directly or indirectly, any action designed, or which will constitute, or has constituted, or might reasonably be expected to cause or result in the stabilization or manipulation of the price of any security of the Partnership to facilitate the sale or resale of the Units;

     (p) to use its reasonable best efforts to cause the Units to be listed on the NYSE and to maintain the listing of the Common Units, including the Units, on the NYSE; and

     (q) to maintain a transfer agent and, if necessary under the jurisdiction of incorporation of the Partnership, a registrar for the Common Units.

     5. Reimbursement of the Underwriters’ Expenses. If, after the execution and delivery of this Agreement, the Units are not delivered for any reason other than the termination of this Agreement pursuant to Section (2)(A), (2)(C), (2)(D) or (2)(E) of the second paragraph of Section 7 hereof, the fifth paragraph of Section 8 hereof or the default by one or more of the Underwriters in its or their respective obligations hereunder, the Partnership shall, in addition to paying the amounts described in Section 4(k) hereof, reimburse the Underwriters for all of their reasonable out-of-pocket expenses, including the fees and disbursements of their counsel.

     6. Conditions of the Underwriters’ Obligations. The several obligations of the Underwriters hereunder are subject to the accuracy of the representations and warranties on the part of the Partnership on the date hereof, at the time of purchase and, if applicable, at the additional time of purchase, the performance by the Partnership of its obligations hereunder and to the following additional conditions precedent:

     (a) The Partnership shall furnish to you at the time of purchase and, if applicable, at the additional time of purchase, an opinion of Vinson & Elkins L.L.P., counsel for the Partnership, addressed to the Underwriters, and dated the time of purchase or the additional time of purchase, as the case may be, with executed copies for each Managing Underwriter, and in form and substance satisfactory to UBS, in the form set forth in Exhibit B hereto.

     (b) The Partnership shall furnish to you at the time of purchase and, if applicable, at the additional time of purchase, an opinion of the General Counsel of the General Partner, addressed to the Underwriters, and dated the time of purchase or the additional time of purchase, as the case may be, with executed copies for each Managing Underwriter, and in form and substance satisfactory to UBS, in the form set forth in Exhibit C hereto.

     (c) You shall have received from Ernst & Young LLP letters dated, respectively, the date of this Agreement, the date of the Prospectus Supplement, the time of purchase and, if applicable, the additional time of purchase, and addressed to the Underwriters (with executed copies for each Managing Underwriter) in the forms satisfactory to UBS, which letters shall cover, without limitation, the various financial disclosures contained in the Registration Statement, the Pre-Pricing Prospectuses, the Prospectus and the Permitted Free Writing Prospectuses, if any.

     (d) You shall have received at the time of purchase and, if applicable, at the additional time of purchase, the favorable opinion of Baker Botts L.L.P., counsel for the Underwriters, dated the time of purchase or the additional time of purchase, as the case may be, in form and substance reasonably satisfactory to UBS.

     (e) No Prospectus or amendment or supplement to the Registration Statement or the Prospectus shall have been filed to which you shall have reasonably objected in writing.

     (f) The Registration Statement and any registration statement required to be filed, prior to the sale of the Units, under the Act pursuant to Rule 462(b) shall have been filed and shall have become effective under the Act. The Prospectus Supplement shall have been filed with the Commission pursuant to Rule 424(b) under the Act at or before 5:30 P.M., New York City time, on the second full business day after the date of this Agreement (or such earlier time as may be required under the Act).

     (g) Prior to and at the time of purchase, and, if applicable, the additional time of purchase, (i) no stop order with respect to the effectiveness of the Registration Statement shall have been issued under the Act or proceedings initiated under Section 8(d) or 8(e) of the Act; (ii) the Registration Statement and all amendments thereto shall not contain an untrue statement of a material fact or omit to state a material fact required to be stated therein or necessary to make the statements therein not misleading; (iii) none of the Pre-Pricing Prospectuses or the Prospectus, and no amendment or supplement thereto, shall include an untrue statement of a material fact or omit to state a material fact necessary in order to make the statements therein, in the light of the circumstances under which they are made, not misleading; (iv) no Disclosure Package, and no amendment or supplement thereto, shall include an untrue statement of a material fact or omit to state a material fact necessary in order to make the statements therein, in the light of the circumstances under which they are made, not misleading; (v) none of the Permitted Free Writing Prospectuses, if any, shall include an untrue statement of a material fact or omit to state a material fact necessary in order to make the statements therein, in the light of the circumstances under which they are made, not misleading; and (vi) since the date of the most recent financial statements included in the Registration Statement, the Pre-Pricing Prospectus and the Prospectus, there has been no Material Adverse Effect, except as set forth in or contemplated in the Registration Statement, the Pre-Pricing Prospectus and the Prospectus.

     (h) The Partnership will, at the time of purchase and, if applicable, at the additional time of purchase, deliver to you a certificate of the Chief Executive Officer and the Chief Financial Officer of the General Partner, dated the time of purchase or the additional time of purchase, as the case may be, in the form attached as Exhibit D hereto.

     (i) You shall have received an agreement for the benefit of the Underwriters in the form set forth as Exhibit A hereto, signed by each director and “officer” (within the meaning of Rule 169-1(f) under the Exchange Act) and each holder of Common Units named in Exhibit A-1 hereto (each a “Lock-Up Agreement”) and each such Lock-Up Agreement shall be in full force and effect at the time of purchase and the additional time of purchase, as the case may be.

     (j) At the time of execution of this Agreement, the Underwriters shall have received from NSAI a letter, in form and substance reasonably satisfactory to the Underwriters, addressed to the Underwriters and dated the date hereof covering the matters described in Exhibit E.

     (k) The Partnership shall have furnished to you such other documents and certificates as to the accuracy and completeness of any statement in the Registration Statement, any Pre-Pricing Prospectus, the Prospectus or any Permitted Free Writing Prospectus as of the time of purchase and, if applicable, the additional time of purchase, as you may reasonably request.

     (l) The Units shall have been approved for listing on the NYSE, subject only to notice of issuance at or prior to the time of purchase or the additional time of purchase, as the case may be.

     (m) FINRA shall not have raised any objection with respect to the fairness or reasonableness of the underwriting, or other arrangements of the transactions, contemplated hereby.

     7. Effective Date of Agreement; Termination. This Agreement shall become effective when the parties hereto have executed and delivered this Agreement.

          The obligations of the several Underwriters hereunder shall be subject to termination in the absolute discretion of the Managing Underwriters, if (1) since the time of execution of this Agreement or the earlier respective dates as of which information is given in the Registration Statement, the Pre-Pricing Prospectuses, the Prospectus and the Permitted Free Writing Prospectuses, if any, there has been any change or any development involving a prospective change in the business, properties, management, financial condition or results of operations of the Partnership Entities taken as a whole, the effect of which change or development is, in the judgment of the Managing Underwriters, so material and adverse as to make it impractical or inadvisable to proceed with the public offering or the delivery of the Units on the terms and in the manner contemplated in the Registration Statement, the Pre-Pricing Prospectuses, the Prospectus and the Permitted Free Writing Prospectuses, if any, or (2) since the time of execution of this Agreement, there shall have occurred: (A) a suspension or material

limitation in trading in securities generally on the NYSE, the NYSE Amex Equities or the NASDAQ; (B) a suspension or material limitation in trading in the Partnership’s securities on the NYSE; (C) a general moratorium on commercial banking activities declared by either federal or New York State authorities or a material disruption in commercial banking or securities settlement or clearance services in the United States; (D) an outbreak or escalation of hostilities or acts of terrorism involving the United States or a declaration by the United States of a national emergency or war; or (E) any other calamity or crisis or any change in financial, political or economic conditions in the United States or elsewhere, if the effect of any such event specified in clause (D) or (E), in the judgment of the Managing Underwriters, makes it impractical or inadvisable to proceed with the public offering or the delivery of the Units on the terms and in the manner contemplated in the Registration Statement, the Pre-Pricing Prospectuses, the Prospectus and the Permitted Free Writing Prospectuses, if any, or (3) since the time of execution of this Agreement, there shall have occurred any downgrading, or any notice or announcement shall have been given or made of: (A) any intended or potential downgrading or (B) any watch, review or possible change that does not indicate an affirmation or improvement in the rating accorded any securities of or guaranteed by the Partnership Entities by any “nationally recognized statistical rating organization,” as that term is defined in Rule 436(g)(2) under the Act.

          If the Managing Underwriters elect to terminate this Agreement as provided in this Section 7, the Partnership and each other Underwriter shall be notified promptly in writing.

          If the sale to the Underwriters of the Units, as contemplated by this Agreement, is not carried out by the Underwriters for any reason permitted under this Agreement, or if such sale is not carried out because the Partnership shall be unable to comply with any of the terms of this Agreement, the Partnership shall not be under any obligation or liability under this Agreement (except to the extent provided in Sections 4(k), 5 and 9 hereof), and the Underwriters shall be under no obligation or liability to the Partnership under this Agreement (except to the extent provided in Section 9 hereof) or to one another hereunder.

     8. Increase in Underwriters’ Commitments. Subject to Sections 6 and 7 hereof, if any Underwriter shall default in its obligation to take up and pay for the Firm Units to be purchased by it hereunder (otherwise than for a failure of a condition set forth in Section 6 hereof or a reason sufficient to justify the termination of this Agreement under the provisions of Section 7 hereof) and if the number of Firm Units which all Underwriters so defaulting shall have agreed but failed to take up and pay for does not exceed 10% of the total number of Firm Units, the non-defaulting Underwriters (including the Underwriters, if any, substituted in the manner set forth below) shall take up and pay for (in addition to the aggregate number of Firm Units they are obligated to purchase pursuant to Section 1 hereof) the number of Firm Units agreed to be purchased by all such defaulting Underwriters, as hereinafter provided. Such Units shall be taken up and paid for by such non-defaulting Underwriters in such amount or amounts as you may designate with the consent of each Underwriter so designated or, in the event no such designation is made, such Units shall be taken up and paid for by all non-defaulting Underwriters pro rata in proportion to the aggregate number of Firm Units set forth opposite the names of such non-defaulting Underwriters in Schedule A.

          Without relieving any defaulting Underwriter from its obligations hereunder, the Partnership agrees with the non-defaulting Underwriters that it will not sell any Firm Units hereunder unless all of the Firm Units are purchased by the Underwriters (or by substituted Underwriters selected by you with the approval of the Partnership or selected by the Partnership with your approval).

          If a new Underwriter or Underwriters are substituted by the Underwriters or by the Partnership for a defaulting Underwriter or Underwriters in accordance with the foregoing provision, the Partnership or you shall have the right to postpone the time of purchase for a period not exceeding five business days in order that any necessary changes in the Registration Statement and the Prospectus and other documents may be effected.

          The term “Underwriter” as used in this Agreement shall refer to and include any Underwriter substituted under this Section 8 with like effect as if such substituted Underwriter had originally been named in Schedule A hereto.

          If the aggregate number of Firm Units which the defaulting Underwriter or Underwriters agreed to purchase exceeds 10% of the total number of Firm Units which all Underwriters agreed to purchase hereunder, and if neither the non-defaulting Underwriters nor the Partnership shall make arrangements within the five business day period stated above for the purchase of all the Firm Units which the defaulting Underwriter or Underwriters agreed to purchase hereunder, this Agreement shall terminate without further act or deed and without any liability on the part of the Partnership to any Underwriter and without any liability on the part of any non-defaulting Underwriter to the Partnership. Nothing in this paragraph, and no action taken hereunder, shall relieve any defaulting Underwriter from liability in respect of any default of such Underwriter under this Agreement.

9. Indemnity and Contribution.

     (a) The Partnership agrees to indemnify, defend and hold harmless each Underwriter, its partners, directors, officers and members, any person who controls any Underwriter within the meaning of Section 15 of the Act or Section 20 of the Exchange Act, UBS AG and UBS Financial Services Inc., and the successors and assigns of all of the foregoing persons, from and against any loss, damage, expense, liability or claim (including the reasonable cost of investigation) which, jointly or severally, any such Underwriter or any such person may incur under the Act, the Exchange Act, the common law or otherwise, insofar as such loss, damage, expense, liability or claim arises out of or is based upon (i) any untrue statement or alleged untrue statement of a material fact contained in the Registration Statement (or in the Registration Statement as amended by any post-effective amendment thereof by the Partnership) or arises out of or is based upon any omission or alleged omission to state a material fact required to be stated therein or necessary to make the statements therein not misleading, except insofar as any such loss, damage, expense, liability or claim arises out of or is based upon any untrue statement or alleged untrue statement of a material fact contained in, and in conformity with information concerning such Underwriter furnished in writing by or on behalf of such Underwriter through you to the Partnership expressly for use in, the Registration Statement or arises out of or is based upon any omission or alleged omission to state a

material fact in the Registration Statement in connection with such information, which material fact was not contained in such information and which material fact was required to be stated in such Registration Statement or was necessary to make such information not misleading or (ii) any untrue statement or alleged untrue statement of a material fact included in any Prospectus (the term Prospectus for the purpose of this Section 9 being deemed to include any Basic Prospectus, any Pre-Pricing Prospectus, the Prospectus Supplement, the Prospectus and any amendments or supplements to the foregoing), in any Covered Free Writing Prospectus, in any “issuer information” (as defined in Rule 433 under the Act) of the Partnership, which “issuer information” is required to be, or is, filed with the Commission, or in any Prospectus together with any combination of one or more of the Covered Free Writing Prospectuses, if any, or arises out of or is based upon any omission or alleged omission to state a material fact necessary in order to make the statements therein, in the light of the circumstances under which they were made, not misleading, except, with respect to such Prospectus or any Permitted Free Writing Prospectus, insofar as any such loss, damage, expense, liability or claim arises out of or is based upon any untrue statement or alleged untrue statement of a material fact contained in, and in conformity with information concerning such Underwriter furnished in writing by or on behalf of such Underwriter through you to the Partnership expressly for use in, such Prospectus or Permitted Free Writing Prospectus or arises out of or is based upon any omission or alleged omission to state a material fact in such Prospectus or Permitted Free Writing Prospectus in connection with such information, which material fact was not contained in such information and which material fact was necessary in order to make the statements in such information, in the light of the circumstances under which they were made, not misleading.

     (b) Each Underwriter severally agrees to indemnify, defend and hold harmless the Partnership, the General Partner, the directors and officers of the General Partner and any person who controls the Partnership within the meaning of Section 15 of the Act or Section 20 of the Exchange Act, and the successors and assigns of all of the foregoing persons, from and against any loss, damage, expense, liability or claim (including the reasonable cost of investigation) which, jointly or severally, the Partnership or any such person may incur under the Act, the Exchange Act, the common law or otherwise, insofar as such loss, damage, expense, liability or claim arises out of or is based upon (i) any untrue statement or alleged untrue statement of a material fact contained in, and in conformity with information concerning such Underwriter furnished in writing by or on behalf of such Underwriter through you to the Partnership expressly for use in, the Registration Statement (or in the Registration Statement as amended by any post-effective amendment thereof by the Partnership), or arises out of or is based upon any omission or alleged omission to state a material fact in such Registration Statement in connection with such information, which material fact was not contained in such information and which material fact was required to be stated in such Registration Statement or was necessary to make such information not misleading or (ii) any untrue statement or alleged untrue statement of a material fact contained in, and in conformity with information concerning such Underwriter furnished in writing by or on behalf of such Underwriter through you to the Partnership expressly for use in, a Prospectus or a Permitted Free Writing Prospectus, or arises out of or is based upon any omission or

alleged omission to state a material fact in such Prospectus or Permitted Free Writing Prospectus in connection with such information, which material fact was not contained in such information and which material fact was necessary in order to make the statements in such information, in the light of the circumstances under which they were made, not misleading.

     (c) If any action, suit or proceeding (each, a “Proceeding”) is brought against a person (an “indemnified party”) in respect of which indemnity may be sought against the Partnership or an Underwriter (as applicable, the “indemnifying party”) pursuant to subsection (a) or (b), respectively, of this Section 9, such indemnified party shall promptly notify such indemnifying party in writing of the institution of such Proceeding and such indemnifying party shall assume the defense of such Proceeding, including the employment of counsel reasonably satisfactory to such indemnified party and payment of all fees and expenses. No indemnification provided for in Section 9(a) or (b) shall be available to any party who shall fail to give notice as provided in this Section 9(c) if the party to whom notice was not given was unaware of the proceeding to which such notice would have related and was materially prejudiced by the failure to give such notice, but the failure to give such notice shall not relieve the indemnifying party or parties from any liability which it or they may have to the indemnified party for contribution or otherwise than on account of the provisions of Section 9(a) or (b). The indemnified party or parties shall have the right to employ its or their own counsel in any such case, but the fees and expenses of such counsel shall be at the expense of such indemnified party or parties unless the employment of such counsel shall have been authorized in writing by the indemnifying party in connection with the defense of such Proceeding or the indemnifying party shall not have, within a reasonable period of time in light of the circumstances, employed counsel to defend such Proceeding or such indemnified party or parties shall have reasonably concluded that there may be defenses available to it or them which are different from, additional to or in conflict with those available to such indemnifying party (in which case such indemnifying party shall not have the right to direct the defense of such Proceeding on behalf of the indemnified party or parties), in any of which events such fees and expenses shall be borne by such indemnifying party and paid as incurred (it being understood, however, that such indemnifying party shall not be liable for the expenses of more than one separate counsel (in addition to any local counsel) in any one Proceeding or series of related Proceedings in the same jurisdiction representing the indemnified parties who are parties to such Proceeding). The indemnifying party shall not be liable for any settlement of any Proceeding effected without its written consent but, if settled with its written consent, such indemnifying party agrees to indemnify and hold harmless the indemnified party or parties from and against any loss or liability by reason of such settlement. No indemnifying party shall, without the prior written consent of the indemnified party, effect any settlement of any pending or threatened Proceeding in respect of which any indemnified party is or could have been a party and indemnity could have been sought hereunder by such indemnified party, unless such settlement includes an unconditional release of such indemnified party from all liability on claims that are the subject matter of such Proceeding and does not include an admission of fault or culpability or a failure to act by or on behalf of such indemnified party.

     (d) If the indemnification provided for in this Section 9 is unavailable to an indemnified party under subsections (a) and (b) of this Section 9 or insufficient to hold an indemnified party harmless in respect of any losses, damages, expenses, liabilities or claims referred to therein, then each applicable indemnifying party shall contribute to the amount paid or payable by such indemnified party as a result of such losses, damages, expenses, liabilities or claims (i) in such proportion as is appropriate to reflect the relative benefits received by the Partnership on the one hand and of the Underwriters on the other in connection with the statements or omissions which resulted in such losses, damages, expenses, liabilities or claims, as well as any other relevant equitable considerations. The relative benefits received by the Partnership on the one hand and the Underwriters on the other shall be deemed to be in the same respective proportions as the total proceeds from the offering (net of underwriting discounts and commissions but before deducting expenses) received by the Partnership, and the total underwriting discounts and commissions received by the Underwriters, bear to the aggregate public offering price of the Units. The relative fault of the Partnership on the one hand and of the Underwriters on the other shall be determined by reference to, among other things, whether the untrue statement or alleged untrue statement of a material fact or omission or alleged omission relates to information supplied by the Partnership or by the Underwriters and the parties’ relative intent, knowledge, access to information and opportunity to correct or prevent such statement or omission. The amount paid or payable by a party as a result of the losses, damages, expenses, liabilities and claims referred to in this subsection shall be deemed to include any legal or other fees or expenses reasonably incurred by such party in connection with investigating, preparing to defend or defending any Proceeding.

     (e) The Partnership and the Underwriters agree that it would not be just and equitable if contribution pursuant to this Section 9 were determined by pro rata allocation (even if the Underwriters were treated as one entity for such purpose) or by any other method of allocation that does not take account of the equitable considerations referred to in subsection (c) above. Notwithstanding the provisions of this Section 9, no Underwriter shall be required to contribute any amount in excess of the amount by which the total price at which the Units underwritten by such Underwriter and distributed to the public were offered to the public exceeds the amount of any damage which such Underwriter has otherwise been required to pay by reason of such untrue statement or alleged untrue statement or omission or alleged omission. No person guilty of fraudulent misrepresentation (within the meaning of Section 11(f) of the Act) shall be entitled to contribution from any person who was not guilty of such fraudulent misrepresentation. The Underwriters’ obligations to contribute pursuant to this Section 9 are several in proportion to their respective underwriting commitments and not joint.

     (f) The indemnity and contribution agreements contained in this Section 9 and the covenants, warranties and representations of the Partnership contained in this Agreement shall remain in full force and effect regardless of any investigation made by or on behalf of any Underwriter, any of their respective partners, directors, officers or members or any person (including each partner, officer, director or member of such person) who controls any Underwriter within the meaning of Section 15 of the Act or

Section 20 of the Exchange Act, or by or on behalf of the Partnership, the directors or officers of the General Partner or any person who controls the Partnership within the meaning of Section 15 of the Act or Section 20 of the Exchange Act, and shall survive any termination of this Agreement or the issuance and delivery of the Units. The Partnership and each Underwriter agree promptly to notify each other of the commencement of any Proceeding against it and, in the case of the Partnership, against any of the General Partner’s officers or directors in connection with the issuance and sale of the Units, or in connection with the Registration Statement, any Basic Prospectus, any Pre-Pricing Prospectus, the Prospectus or any Permitted Free Writing Prospectus.

     10. Information Furnished by the Underwriters. The statements set forth in the last paragraph on the cover page of the Prospectus and the statements set forth in the fourth, thirteenth, fourteenth, fifteenth, sixteenth and eighteenth paragraphs under the caption “Underwriting” in the Prospectus, only insofar as such statements relate to the amount of selling concession and reallowance or to over-allotment and stabilization activities that may be undertaken by the Underwriters, constitute the only information furnished by or on behalf of the Underwriters, as such information is referred to in Sections 3 and 9 hereof.

     11. Notices. Except as otherwise herein provided, all statements, requests, notices and agreements shall be in writing or by telegram or facsimile and, if to the Underwriters, shall be sufficient in all respects if delivered or sent to UBS Securities LLC, 299 Park Avenue, New York, NY 10171-0026, Attention: Syndicate Department; and if to the Partnership, shall be sufficient in all respects if delivered or sent to the Partnership at the offices of the Partnership at Pioneer Southwest Energy Partners L.P., 5205 N. O’Connor Blvd., Suite 200, Irving, Texas 75039 (facsimile: 972 ###-###-####), Attention: General Counsel.

     12. Governing Law; Construction. This Agreement and any claim, counterclaim or dispute of any kind or nature whatsoever arising out of or in any way relating to this Agreement (“Claim”), directly or indirectly, shall be governed by, and construed in accordance with, the laws of the State of New York. The section headings in this Agreement have been inserted as a matter of convenience of reference and are not a part of this Agreement.

     13. Submission to Jurisdiction. Except as set forth below, no Claim may be commenced, prosecuted or continued in any court other than the courts of the State of New York located in the City and County of New York or in the United States District Court for the Southern District of New York, which courts shall have exclusive jurisdiction over the adjudication of such matters, and the Partnership consents to the jurisdiction of such courts and personal service with respect thereto. The Partnership hereby consents to personal jurisdiction, service and venue in any court in which any Claim arising out of or in any way relating to this Agreement is brought by any third party against any Underwriter or any indemnified party. Each Underwriter and the Partnership (on its behalf and, to the extent permitted by applicable law, on behalf of the holders of its Common Units and its affiliates) each waive all right to trial by jury in any action, proceeding or counterclaim (whether based upon contract, tort or otherwise) in any way arising out of or relating to this Agreement. The Partnership agrees that a final judgment in any such action, proceeding or counterclaim brought in any such court shall be conclusive and binding upon the Partnership and may be enforced in any other courts to the jurisdiction of which

the Partnership is or may be subject, by suit upon such judgment.

     14. Parties at Interest. The Agreement herein set forth has been and is made solely for the benefit of the Underwriters and the Partnership and to the extent provided in Section 9 hereof the controlling persons, partners, directors, officers, members and affiliates referred to in such Section, and their respective successors, assigns, heirs, personal representatives and executors and administrators. No other person, partnership, association or corporation (including a purchaser, as such purchaser, from any of the Underwriters) shall acquire or have any right under or by virtue of this Agreement.

     15. No Fiduciary Relationship. The Partnership hereby acknowledges that the Underwriters are acting solely as underwriters in connection with the purchase and sale of the Partnership’s securities. The Partnership further acknowledges that the Underwriters are acting pursuant to a contractual relationship created solely by this Agreement entered into on an arm’s length basis, and in no event do the parties intend that the Underwriters act or be responsible as a fiduciary to the Partnership, its management, stockholders or creditors or any other person in connection with any activity that the Underwriters may undertake or have undertaken in furtherance of the purchase and sale of the Partnership’s securities, either before or after the date hereof. The Underwriters hereby expressly disclaim any fiduciary or similar obligations to the Partnership, either in connection with the transactions contemplated by this Agreement or any matters leading up to such transactions, and the Partnership hereby confirms its understanding and agreement to that effect. The Partnership and the Underwriters agree that they are each responsible for making their own independent judgments with respect to any such transactions and that any opinions or views expressed by the Underwriters to the Partnership regarding such transactions, including, but not limited to, any opinions or views with respect to the price or market for the Partnership’s securities, do not constitute advice or recommendations to the Partnership. The Partnership and the Underwriters agree that the Underwriters are acting as principal and not the agent or fiduciary of the Partnership with respect to the transactions contemplated hereby or the process leading thereto (irrespective of whether any Underwriter has advised or is currently advising the Partnership on other matters). The Partnership hereby waives and releases, to the fullest extent permitted by law, any claims that the Partnership may have against the Underwriters with respect to any breach or alleged breach of any fiduciary, advisory or similar duty to the Partnership in connection with the transactions contemplated by this Agreement or any matters leading up to such transactions.

     16. Counterparts. This Agreement may be signed by the parties in one or more counterparts which together shall constitute one and the same agreement among the parties.

     17. Successors and Assigns. This Agreement shall be binding upon the Underwriters and the Partnership and its successors and assigns and any successor or assign of any substantial portion of the Partnership’s and any of the Underwriters’ respective businesses and/or assets.

     18. Miscellaneous. UBS, an indirect, wholly owned subsidiary of UBS AG, is not a bank and is separate from any affiliated bank, including any U.S. branch or agency of UBS AG. Because UBS is a separately incorporated entity, it is solely responsible for its own contractual obligations and commitments, including obligations with respect to sales and purchases of

securities. Securities sold, offered or recommended by UBS are not deposits, are not insured by the Federal Deposit Insurance Corporation, are not guaranteed by a branch or agency, and are not otherwise an obligation or responsibility of a branch or agency.

[The Remainder of This Page Intentionally Left Blank; Signature Page Follows]

          If the foregoing correctly sets forth the understanding among the Partnership and the several Underwriters, please so indicate in the space provided below for that purpose, whereupon this Agreement and your acceptance shall constitute a binding agreement among the Partnership and the Underwriters, severally.

Signature Page to Underwriting Agreement

Accepted and agreed to as of the date
first above written, on behalf of
themselves and the other several
Underwriters named in Schedule A

UBS Securities LLC
Merrill Lynch, Pierce, Fenner & Smith Incorporated
Citigroup Global Markets Inc.
Wells Fargo Securities, LLC

Signature Page to Underwriting Agreement

SCHEDULE A

		Number of
Underwriters		Firm Units
UBS SECURITIES LLC.			607,500
MERRILL LYNCH, PIERCE, FENNER & SMITH INCORPORATED			607,500
CITIGROUP GLOBAL MARKETS INC.			607,500
WELLS FARGO SECURITIES, LLC.			607,500
J.P. MORGAN SECURITIES INC.			135,000
RBC CAPITAL MARKETS CORPORATION			135,000
Total			2,700,000

SCHEDULE B

None.

SCHEDULE C

Partnership Entity:		Equity Owned By:
Partnership		0.1% general partner interest owned by the General Partner
		Pioneer USA owns 20,521,200 Common Units
		Public unitholders own 8,250,000 Common Units
General Partner		100% of membership interests owned by Pioneer USA
Operating Company		100% of membership interests owned by the Partnership
PSE Finance		100% of capital stock owned by the Partnership

EXHIBIT A

Lock-Up Agreement

November                     , 2009

UBS Securities LLC
Together with the other Underwriters
named in Schedule A to the Underwriting Agreement
referred to herein

c/o UBS Securities LLC
299 Park Avenue
New York, New York 10171-0026

Ladies and Gentlemen:

          This letter is being delivered to you in connection with the proposed Underwriting Agreement (the “Underwriting Agreement”), by and among Pioneer Southwest Energy Partners L.P. (the “Partnership”), Pioneer Natural Resources GP LLC and each of you as representatives of a group of Underwriters named therein, relating to an underwritten public offering of common units, representing limited partner interests in the Partnership (the “Partnership Units”). In order to induce you and the other Underwriters to enter into the Underwriting Agreement, the undersigned will not, without the prior written consent of UBS Securities LLC, Merrill Lynch, Pierce, Fenner & Smith Incorporated, Citigroup Global Markets Inc. and Wells Fargo Securities, LLC offer, sell, contract to sell, pledge or otherwise dispose of (or enter into any transaction which is designed to, or might reasonably be expected to, result in the disposition (whether by actual disposition or effective economic disposition due to cash settlement or otherwise) by the undersigned or any affiliate of the undersigned or any person in privity with the undersigned or any affiliate of the undersigned), directly or indirectly, including the filing (or participation in the filing) of a registration statement with the Securities and Exchange Commission in respect of, or establish or increase a put equivalent position or liquidate or decrease a call equivalent position within the meaning of Section 16 of the Securities Exchange Act of 1934, as amended, and the rules and regulations of the Securities and Exchange Commission promulgated thereunder with respect to, any Partnership Units or any securities convertible into, or exercisable or exchangeable for such Partnership Units, or publicly announce an intention to effect any such transaction, for a period of 60 days after the date of the Underwriting Agreement. The foregoing sentence shall not apply to (i) bona fide gifts, sales or other dispositions of any Partnership Units that are made exclusively between and among the undersigned or members of the undersigned’s family, or affiliates of the undersigned, including its partners (if a partnership) or members (if a limited liability company); provided that it shall be a condition to any such transfer that the transferee/donee agrees to be bound by the terms of the lock-up agreement (including, without limitation, the restrictions set forth in the preceding sentence) to the same extent as if the transferee/donee were a party hereto, and (ii) a maximum of 100,000 Partnership Units in the aggregate that are sold or otherwise disposed of by all of the individuals executing similar letters as this, including the undersigned, in connection with the Underwriting

Agreement. Additionally, Pioneer Natural Resources GP LLC shall be allowed to (i) issue and sell common units pursuant to any employee benefit plan of the Partnership or itself in effect at the date of the closing of the underwritten public offering of the Partnership’s common units (including, without limitation, the forfeiture to the Partnership by the undersigned of Partnership Units in satisfaction of tax withholding obligations arising in connection with the issuance, vesting or exercise of an award under any such plan or the lapse of restrictions thereon), (ii) issue common units issuable upon the conversion of securities or the exercise of warrants outstanding as of the date hereof and (iii) issue common units as consideration for any merger and acquisition made by the Partnership; provided, however, that in the case of any issuance pursuant to this clause (iii), in the case of any merger or acquisition other than the acquisition of all of the outstanding equity of a publicly traded entity, each recipient of any such common units shall sign and deliver an agreement in substantially the form hereof and has been in compliance with the terms hereof since the date first written above as if it had been an original party hereto.

          If for any reason the Underwriting Agreement shall be terminated prior to the “time of purchase” (as defined in the Underwriting Agreement), the agreement set forth above shall likewise be terminated.

		Very truly yours,
		Name:
		Title:

EXHIBIT A-1

LIST OF PARTIES TO EXECUTE LOCK-UP AGREEMENTS

Name		Position
1. Scott D. Sheffield		Chairman of the Board of Directors and Chief Executive Officer
2. Richard P. Dealy		Executive Vice President, Chief Financial Officer, Treasurer and Director
3. Timothy L. Dove		President and Chief Operating Officer
4. Mark S. Berg		Executive Vice President, General Counsel
5. Chris J. Cheatwood		Executive Vice President, Geoscience
6. William F. Hannes		Executive Vice President, Business Development
7. Danny L. Kellum		Executive Vice President, Operations
8. Frank W. Hall		Vice President and Chief Accounting Officer
9. Alan L. Gosule		Director
10. Royce W. Mitchell		Director
11. Arthur L. Smith		Director
12. Phillip A. Gobe		Director
13. Pioneer Natural Resources USA, Inc.

EXHIBIT E

FORM OF ENGINEERS’ RESERVE REPORT LETTER

          This letter was prepared pursuant to Section 6(j) of the Underwriting Agreement dated November 11, 2009, by and among Pioneer Southwest Energy Partners L.P. (the “Partnership”), Pioneer Natural Resources GP LLC and UBS Securities LLC acting as the representatives of the several underwriters listed on Schedule A thereto (collectively, the “Underwriters”) for an offering by the Partnership of an aggregate of 2,700,000 common units, each representing a limited partner interest in the Partnership (the “Units”). The Units are being offered pursuant to the Partnership’s preliminary prospectus dated November 10, 2009 and final prospectus dated the date hereof. We have prepared an audit letter dated February 6, 2009, in which we estimated the proved reserves and future revenue of the Partnership as of December 31, 2008 (the “Audit Letter”).

          1. As of the date of this letter and as of the date of the Audit Letter, we are independent petroleum engineers with respect to the Partnership. We are familiar with the standards pertaining to the estimating and auditing of oil and gas reserve information promulgated by the Securities and Exchange Commission (the “SEC”). Neither we, nor to our knowledge, any of our employees, officers or directors, own interests in the Partnership or any oil and gas properties covered by the Audit Letter. We have not been employed by the Partnership on a contingent basis.

          2. All terms used in this letter, where applicable, conform to the definitions set forth in Rule 4-10 of Regulation S-X promulgated by the SEC.

          3. In our opinion, the information relating to estimated proved reserves, estimated future net revenue from proved reserves and present worth of estimated future net revenue from proved reserves contained in the Audit Letter has been prepared in a manner consistent with the standards and definitions pertaining to the estimation and auditing of oil and gas reserve information promulgated by the SEC. Specifically, such information has been prepared in accordance with Paragraphs 10-13, 15 and 30(a)-(b) of Statement of Financial Accounting Standards No. 69 (November 1982) of the FASB and Rules 4-10(a)(1)-(13) of Regulation S-X and Rule 302(b) of Regulation S-K of the SEC; provided, however, future income tax expenses have not been taken into account in estimating the future net revenue and present worth values set forth in the Audit Letter.

          To the extent the above-enumerated rules, regulations and statements require determinations of an accounting or legal nature or information beyond the scope of the Audit Letter, we are necessarily unable to express an opinion as to whether the above-described information is in accordance therewith or sufficient therefor.

          4. The Audit Letter attached hereto as Appendix 1 is the correct and complete copy of our Audit Letter provided to the Partnership on February 9, 2009.

          5. Although we were not requested to review any subsequent data concerning either the performance of the wells or field operations as of the date hereof, no additional information has been brought to our attention that would lead us to believe that there would be a material change in the estimates of proved reserves or future net revenues from proved reserves attributable to the Partnership’s interests in certain oil and gas properties covered by the Audit Letter, notwithstanding those changes that would result from a change in product prices and reductions for production since January 1, 2009.

          6. You may rely upon the Audit Letter in the same manner as if such letter were addressed to you.

APPENDIX 1 to EXHIBIT E

AUDIT LETTER