Exhibit 1.1

MARKWEST ENERGY PARTNERS, L.P.

2,900,000 COMMON UNITS
REPRESENTING LIMITED PARTNER INTERESTS

UNDERWRITING AGREEMENT

June 5, 2009

June 5, 2009

Morgan Stanley & Co. Incorporated

Ladies and Gentlemen:

MarkWest Energy Partners, L.P., a Delaware limited partnership (the “Partnership”), proposes to issue and sell to Morgan Stanley & Co. Incorporated (the “Underwriter”) 2,900,000 common units (the “Firm Units”) representing limited partner interests in the Partnership (“Common Units”).  The Partnership also proposes to issue and sell to the Underwriter not more than the number of additional Common Units set forth in Schedule I hereto (the “Additional Units”) if and to the extent that you shall have determined to exercise the right to purchase such Additional Units granted to you pursuant to Section 2 of this agreement (the “Agreement”).  The Firm Units and the Additional Units, if purchased, are hereinafter collectively referred to as the “Units.”

The Partnership owns all of the outstanding capital stock of MarkWest Hydrocarbon, Inc., a Delaware corporation (“MarkWest Hydrocarbon”), and a 1% limited liability company interest in MarkWest Energy GP, L.L.C., a Delaware limited liability company (“MarkWest Energy GP”), which serves as the sole general partner of the Partnership.  MarkWest Hydrocarbon owns a 99% limited liability company interest in MarkWest Energy GP and a 100% limited liability company interest in Mason Pipeline Limited Liability Company, a Michigan limited liability company (“Mason LLC”).  The Partnership conducts its business through MarkWest Energy Operating Company, L.L.C., a Delaware limited liability company (the “Operating Company” and, together with the Partnership, the “MarkWest Parties”), MarkWest Hydrocarbon and Mason LLC.  The Operating Company conducts its business through the Operating Subs (as hereinafter defined).  Basin Pipeline L.L.C., a Michigan limited liability company (“Basin LLC”), Bright Star Partnership, a Texas general partnership (“Bright Star”), Centrahoma Processing LLC, a Delaware limited liability company (“Centrahoma LLC”), MarkWest Blackhawk, L.L.C., a Texas limited liability company (“Blackhawk LLC”), MarkWest Energy Appalachia, L.L.C., a Delaware limited liability company (“Appalachia LLC”), MarkWest Energy East Texas Gas Company, L.L.C., a Delaware limited liability company (“East Texas LLC”), MarkWest Gas Marketing, L.L.C., a Texas limited liability company (“Gas Marketing LLC”), MarkWest Gas Services, L.L.C., a Texas limited liability company (“Gas Services LLC”), MarkWest Javelina Company, L.L.C., a Texas limited liability company (“Javelina Company LLC”), MarkWest Javelina Pipeline Company, L.L.C., a Texas limited liability company (“Javelina Pipeline LLC”), MarkWest Liberty Gas Gathering, L.L.C., a Delaware limited liability company (“Liberty LLC”), MarkWest Liberty Midstream & Resources, L.L.C., a Delaware limited liability company (“Liberty Joint Venture”), MarkWest Marketing, L.L.C., a Delaware limited liability company (“Marketing LLC”), MarkWest McAlester, L.L.C., an Oklahoma limited liability company (“McAlester LLC”), MarkWest Michigan Pipeline Company, L.L.C., a Michigan limited liability company (“Michigan Pipeline LLC”), MarkWest New Mexico, L.L.C., a Texas limited liability company (“New Mexico LLC”), MarkWest Pinnacle, L.L.C., a Texas limited liability company (“Pinnacle LLC”), MarkWest Pioneer, L.L.C., a Delaware limited liability company (“Pioneer LLC”), MarkWest Pipeline Company, L.L.C., a Texas limited liability company (“Pipeline Company LLC”), MarkWest PNG Utility, L.L.C., a Texas limited liability company

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(“PNG Utility LLC”), MarkWest Power Tex, L.L.C., a Texas limited liability company (“Power Tex LLC”), MarkWest Texas PNG Utility, L.L.C., a Texas limited liability company (“Texas PNG LLC”), MarkWest Oklahoma Gas Company, L.L.C., an Oklahoma limited liability company (“Oklahoma LLC”), Matrex, L.L.C., a Michigan limited liability company (“Matrex LLC”), Starfish Pipeline Company, L.L.C., a Delaware limited liability company (“Starfish LLC”), West Shore Processing Company, L.L.C., a Michigan limited liability company (“West Shore LLC”) and Wirth Gathering, an Oklahoma general partnership (“Wirth Gathering”), are collectively referred to as the “Operating Subs.”  The Partnership, MarkWest Hydrocarbon, Mason LLC, MarkWest Energy GP, the Operating Company and the Operating Subs are collectively referred to as the “MarkWest Entities.”

1.     Representations and Warranties of the MarkWest Parties.  The MarkWest Parties jointly and severally represent and warrant to and agree with the Underwriter that:

(a)           Registration Statement, Preliminary Prospectus and Prospectus.  A registration statement on Form S-3 relating to the Units (i) has been prepared by the Partnership in conformity with the requirements of the Securities Act of 1933, as amended (the “Securities Act”), and the rules and regulations (the “Rules and Regulations”) of the Securities and Exchange Commission (the “Commission”) thereunder; (ii) has been filed with the Commission under the Securities Act; and (iii) is effective under the Securities Act.  Copies of such registration statement and any amendment thereto have been delivered by the Partnership to you.  As used in this Agreement:

(i)            “Applicable Time” means 8:52 a.m., New York City time, on June 5, 2009, which the Underwriter has informed the Partnership and its counsel is a time prior to the first sale of the Units;

(ii)           “Effective Date” means any date as of which any part of such registration statement relating to the Units became, or is deemed to have become, effective under the Securities Act in accordance with the Rules and Regulations;

(iii)          “Issuer Free Writing Prospectus” means each “free writing prospectus” (as defined in Rule 405 of the Rules and Regulations) prepared by or on behalf of the Partnership or used or referred to by the Partnership in connection with the offering of the Units;

(iv)          “Preliminary Prospectus” means any preliminary prospectus relating to the Units included in such registration statement or filed with the Commission pursuant to Rule 424(b) of the Rules and Regulations, including any preliminary prospectus supplement thereto relating to the Units;

(v)           “Pricing Disclosure Package” means, as of the Applicable Time, the most recent Preliminary Prospectus, together with each Issuer Free Writing Prospectus filed or used by the Partnership on or before the Applicable Time, as set forth on Schedule I hereto, other than a road show that is an Issuer Free Writing Prospectus under Rule 433 of the Rules and Regulations;

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(vi)          “Prospectus” means the final prospectus relating to the Units, including any prospectus supplement thereto relating to the Units, as filed with the Commission pursuant to Rule 424(b) of the Rules and Regulations; and

(vii)         “Registration Statement” means the registration statement on Form S-3 (File No. 333-157883), as amended as of the Effective Date, including any Preliminary Prospectus or the Prospectus and all exhibits to such registration statement.

Any reference to any Preliminary Prospectus or the Prospectus shall be deemed to refer to and include any documents incorporated by reference therein pursuant to Form S-3 under the Securities Act as of the date of such Preliminary Prospectus or the Prospectus, as the case may be.  Any reference to the “most recent Preliminary Prospectus” shall be deemed to refer to the latest Preliminary Prospectus included in the Registration Statement or filed pursuant to Rule 424(b) of the Rules and Regulations prior to or on the date hereof (including, for purposes hereof, any documents incorporated by reference therein prior to or on the date hereof).  Any reference to any amendment or supplement to any Preliminary Prospectus or the Prospectus shall be deemed to refer to and include any document filed under the Securities Exchange Act of 1934, as amended (the “Exchange Act”), after the date of such Preliminary Prospectus or the Prospectus, as the case may be, and incorporated by reference in such Preliminary Prospectus or the Prospectus, as the case may be; and any reference to any amendment to the Registration Statement shall be deemed to include any annual report of the Partnership on Form 10-K filed with the Commission pursuant to Section 13(a) or 15(d) of the Exchange Act after the Effective Date that is incorporated by reference in the Registration Statement.  The Commission has not issued any order preventing or suspending the use of any Preliminary Prospectus or the Prospectus or suspending the effectiveness of the Registration Statement, and no proceeding or examination for such purpose has been instituted or threatened by the Commission.  The Commission has not notified the Partnership of any objection to the use of the form of the Registration Statement.

(b)           Use of Form S-3.  The Partnership meets the requirements for use of Form S-3 under the Securities Act.  As of the Effective Date and the date of this Agreement, the Partnership met, and as of the applicable Closing Date, the Partnership will meet, the requirements of Form S-3 pursuant to the standards for that form prior to October 21, 1992.  Without limiting the generality of the foregoing, as of such dates, the Partnership has been or will be timely in meeting its reporting obligations under the Exchange Act during the immediately preceding 12 months and has or will have an aggregate market capitalization held by non-affiliates of greater than $150 million.  The Partnership is not an “ineligible issuer” in connection with the offering of the Units pursuant to Rules 164, 405 and 433 under the Securities Act.  The Registration Statement has become effective; no stop order suspending the effectiveness of the Registration Statement is in effect, and no proceedings for such purpose have been instituted or are pending before or, to the knowledge of the Partnership, threatened by the Commission.  The Partnership has complied to the Commission’s satisfaction with all requests of the Commission for additional or supplemental information.

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(c)           Registration Statement Conforms to Requirements of Securities Act.  The Registration Statement conformed and will conform in all material respects on the Effective Date and on the applicable Closing Date, and any amendment to the Registration Statement filed after the date hereof will conform in all material respects when filed, to the requirements of the Securities Act and the Rules and Regulations.  The most recent Preliminary Prospectus conformed, and the Prospectus will conform, in all material respects when filed with the Commission pursuant to Rule 424(b) and on the applicable Closing Date to the requirements of the Securities Act and the Rules and Regulations.  The documents incorporated by reference in any Preliminary Prospectus or the Prospectus conformed, and any further documents so incorporated will conform, when filed with the Commission, in all material respects to the requirements of the Exchange Act or the Securities Act, as applicable, and the Rules and Regulations.

(d)           No Omissions or Material Misstatements in Registration Statement.  The Registration Statement did not, as of its most recent Effective Date, 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; provided that no representation or warranty is made as to information contained in or omitted from the Registration Statement in reliance upon and in conformity with written information furnished to the Partnership by the Underwriter specifically for inclusion therein.

(e)           No Omissions or Material Misstatements in Prospectus.  The Prospectus will not, as of its date and on the applicable Closing Date, contain 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 that no representation or warranty is made as to information contained in or omitted from the Prospectus in reliance upon and in conformity with written information furnished to the Partnership by the Underwriter specifically for inclusion therein.

(f)            No Omissions or Material Misstatements in Documents Incorporated by Reference.  The documents incorporated by reference in the Registration Statement, any Preliminary Prospectus or the Prospectus did not, and any further documents filed and incorporated by reference therein will not, when filed with the Commission, contain 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.

(g)           No Omissions or Material Misstatements in Pricing Disclosure Package.  The Pricing Disclosure Package did not, as of the Applicable Time, contain 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 that no representation or warranty is made as to information contained in or omitted from the Pricing Disclosure Package in reliance upon and in conformity with written information furnished to the Partnership by the Underwriter specifically for inclusion therein.

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(h)           No Omissions or Material Misstatements in Free Writing Prospectus. Each Issuer Free Writing Prospectus (including, without limitation, any road show that is a free writing prospectus under Rule 433 of the Rules and Regulations), when considered together with the Pricing Disclosure Package as of the Applicable Time, did not contain 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 that no representation or warranty is made as to information contained in or omitted from any Issuer Free Writing Prospectus (including, without limitation, any road show that is a free writing prospectus under Rule 433 of the Rules and Regulations) in reliance upon and in conformity with written information furnished to the Partnership by the Underwriter specifically for inclusion therein.

(i)            Free Writing Prospectus Conforms to Requirements of Securities Act.  Each Issuer Free Writing Prospectus conformed or will conform in all material respects to the requirements of the Securities Act and the Rules and Regulations on the date of first use, and the Partnership has complied with any filing requirements applicable to such Issuer Free Writing Prospectus pursuant to the Rules and Regulations.  The Partnership has not made any offer relating to the Units that would constitute an Issuer Free Writing Prospectus without the prior written consent of the Underwriter.  The Partnership has retained in accordance with the Rules and Regulations all Issuer Free Writing Prospectuses that were not required to be filed pursuant to the Rules and Regulations.  The Partnership filed the Registration Statement before using any Issuer Free Writing Prospectus and each Issuer Free Writing Prospectus was accompanied by the most recent Preliminary Prospectus satisfying the requirements of Section 10 of the Securities Act.  The Partnership has taken all actions necessary so that any road show (as defined in Rule 433 of the Rules and Regulations) in connection with the offering of the Units will not be required to be filed pursuant to the Rules and Regulations.

(j)            Formation and Due Qualification of the Partnership.  The Partnership has been duly formed and is validly existing in good standing as a limited partnership under the Delaware Revised Uniform Limited Partnership Act (the “Delaware LP Act”) with full partnership power and authority to own or lease its properties and to conduct its business in all material respects as described in the Pricing Disclosure Package and the Prospectus.  The Partnership is duly registered or qualified as a foreign limited partnership for the transaction of business under the laws of each jurisdiction in which the character of the business conducted by it or the nature or location of the properties owned or leased by it makes such registration or qualification necessary, except where the failure so to register or qualify would not, individually or in the aggregate, (i) have a material adverse effect on the business, prospects, financial condition or results of operations of the MarkWest Entities, taken as a whole (“Material Adverse Effect”), or (ii) subject the limited partners of the Partnership to any material liability or disability.

(k)           Formation and Due Qualification of MarkWest Hydrocarbon.  MarkWest Hydrocarbon has been duly formed and is validly existing in good standing as a corporation under the Delaware General Corporation Law (the “DGCL”), with full corporate power and authority to own or lease its properties and to conduct its business, in each case in all material respects as described in the Pricing Disclosure Package and

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the Prospectus.  MarkWest Hydrocarbon is duly registered or qualified as a foreign corporation for the transaction of business under the laws of each jurisdiction in which the character of the business conducted by it or the nature or location of the properties owned or leased by it makes such registration or qualification necessary, except where the failure so to register or qualify would not, individually or in the aggregate, have a Material Adverse Effect.

(l)            Formation and Due Qualification of Limited Liability Companies.  Each of MarkWest Energy GP, the Operating Company, Centrahoma LLC, Appalachia LLC, East Texas LLC, Liberty LLC, Liberty Joint Venture, Marketing LLC, Pioneer LLC and Starfish LLC has been duly formed and is validly existing in good standing as a limited liability company under the Delaware Limited Liability Company Act (the “Delaware LLC Act”); each of Blackhawk LLC, Gas Marketing LLC, Gas Services LLC, Javelina Company LLC, Javelina Pipeline LLC, New Mexico LLC, Pinnacle LLC, Pipeline Company LLC, PNG Utility LLC, Power Tex LLC and Texas PNG LLC has been duly formed and is validly existing and in good standing as a limited liability company under the Texas Limited Liability Company Act (the “Texas LLC Act”) or the Texas Business Organizations Code (the “TBOC”), as applicable; each of Basin LLC, Mason LLC, Matrex LLC, Michigan Pipeline LLC and West Shore LLC has been duly formed and is validly existing and in good standing as a limited liability company under the Michigan Limited Liability Company Act (the “Michigan LLC Act”); and each of McAlester LLC and Oklahoma LLC has been duly formed and is validly existing and in good standing as a limited liability company under the Oklahoma Limited Liability Company Act (the “Oklahoma LLC Act”), in each case with full limited liability company power and authority to own or lease its properties and to conduct its business, in each case in all material respects as described in the Pricing Disclosure Package and the Prospectus.  Each of MarkWest Energy GP, the Operating Company and each of the Operating Subs set forth above is duly registered or qualified as a foreign limited liability company for the transaction of business under the laws of each jurisdiction in which the character of the business conducted by it or the nature or location of the properties owned or leased by it makes such registration or qualification necessary, except where the failure so to register or qualify would not, individually or in the aggregate, have a Material Adverse Effect.

(m)          Formation and Qualification of Bright Star.  Bright Star has been duly formed and is validly existing as a general partnership under the laws of the State of Texas with full partnership power and authority to own or lease its properties and to conduct its business, in each case in all material respects as described in the Pricing Disclosure Package and the Prospectus.  Bright Star is duly registered or qualified as a foreign partnership for the transaction of business under the laws of each jurisdiction in which the character of the business conducted by it or the nature or location of the properties owned or leased by it makes such registration or qualification necessary, except where the failure so to register or qualify would not, individually or in the aggregate, have a Material Adverse Effect.

(n)           Formation and Qualification of Wirth Gathering.  Wirth Gathering has been duly formed and is validly existing as a general partnership under the laws of the

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State of Oklahoma with full partnership power and authority to own or lease its properties and to conduct its business, in each case in all material respects as described in the Pricing Disclosure Package and the Prospectus.  Wirth Gathering is duly registered or qualified as a foreign partnership for the transaction of business under the laws of each jurisdiction in which the character of the business conducted by it or the nature or location of the properties owned or leased by it makes such registration or qualification necessary, except where the failure so to register or qualify would not, individually or in the aggregate, have a Material Adverse Effect.

(o)           General Partner of the Partnership.  MarkWest Energy GP is the sole general partner of the Partnership with all necessary limited liability company power and authority to act as the general partner of the Partnership.

(p)           Capitalization.  The issued and outstanding limited partner interests of the Partnership consist of 56,893,885 Common Units and 22,640,000 Class A Units, as such terms are defined in the Third Amended and Restated Agreement of Limited Partnership of the Partnership (as the same has been and may be amended or restated at or prior to the applicable Closing Date, the “Partnership Agreement”).  All outstanding Common Units and Class A Units and the limited partner interests 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 Sections 17-607 and 17-804 of the Delaware LP Act and as otherwise described in the Prospectus).  MarkWest Hydrocarbon owns 8,900,000 Class A Units and (ii) MarkWest Energy GP owns 13,740,000 Class A Units, in each case free and clear of all liens, encumbrances, security interests, equities, charges or claims, except as may arise under the Credit Agreement, dated as of February 20, 2008 (as amended by the First Amendment thereto dated January 28, 2009, the “Credit Agreement”), among the Partnership, Royal Bank of Canada, as administrative and collateral agent, and the other parties named therein, or as described in the Pricing Disclosure Package and the Prospectus.

(q)           Due Authorization and Valid Issuance of Units.  At the applicable Closing Date, the Units to be sold by the Partnership and the limited partner interests represented thereby will be duly authorized in accordance with the Partnership Agreement and, when issued and delivered to the Underwriter against payment therefor in accordance with the terms hereof, will be validly issued, fully paid (to the extent required under the Partnership Agreement) and nonassessable (except as such nonassessability may be affected by Sections 17-607 and 17-804 of the Delaware LP Act).

(r)            Ownership of MarkWest Hydrocarbon.  The Partnership owns 100% of the issued and outstanding shares of capital stock of MarkWest Hydrocarbon; such shares have been duly authorized and validly issued in accordance with the certificate of incorporation and bylaws of MarkWest Hydrocarbon and are fully paid and nonassessable; and the Partnership owns such shares free and clear of all liens, encumbrances, security interests, equities, charges or claims, except as may arise under the Credit Agreement or as described in the Pricing Disclosure Package and the Prospectus.

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(s)           Ownership of the Membership Interests in MarkWest Energy GP and the Operating Company by the Partnership.  The Partnership owns 1% of the issued and outstanding membership interests in MarkWest Energy GP and 100% of the issued and outstanding membership interests in the Operating Company; such membership interests have been duly authorized and validly issued in accordance with the limited liability company agreement of MarkWest Energy GP (as the same may be amended or restated at or prior to the applicable Closing Date, the “MarkWest Energy GP LLC Agreement”) and the limited liability company agreement of the Operating Company (as the same may be amended or restated at or prior to the applicable Closing Date, the “Operating Company LLC Agreement”), respectively, and are fully paid (to the extent required under the MarkWest Energy GP LLC Agreement and the Operating Company LLC Agreement, respectively) and nonassessable (except in each case as such nonassessability may be affected by Sections 18-607 and 18-804 of the Delaware LLC Act); and the Partnership owns such membership interests free and clear of all liens, encumbrances, security interests, equities, charges or claims, except as may arise under the Credit Agreement or as described in the Pricing Disclosure Package and the Prospectus.

(t)            Ownership of the Membership Interests in MarkWest Energy GP and Mason LLC by MarkWest Hydrocarbon.  MarkWest Hydrocarbon owns 99% of the issued and outstanding membership interests in MarkWest Energy GP and 100% of the issued and outstanding membership interests in Mason LLC; such membership interests have been duly authorized and validly issued in accordance with the MarkWest Energy GP LLC Agreement and the organizational documents of Mason LLC, respectively, and are fully paid (to the extent required under the MarkWest Energy GP LLC Agreement and the organizational documents of Mason LLC, respectively) and nonassessable (except as such nonassessability may be affected by Sections 18-607 and 18-804 of the Delaware LLC Act or Section  ###-###-#### of the Michigan LLC Act); and MarkWest Hydrocarbon owns its membership interests free and clear of all liens, encumbrances, security interests, equities, charges or claims, except as may arise under the Credit Agreement or as described in the Pricing Disclosure Package and the Prospectus.

(u)           Ownership of the LLC Operating Subs.  (i) The Operating Company directly owns 100% of the issued and outstanding membership interests in Basin LLC, Blackhawk LLC, Appalachia LLC, East Texas LLC, Gas Marketing LLC, Gas Services LLC, Javelina Company LLC, Javelina Pipeline LLC, Liberty LLC, Marketing LLC, Michigan Pipeline LLC, New Mexico LLC, Pinnacle LLC, Pipeline Company LLC, PNG Utility LLC, Power Tex LLC, Texas PNG LLC, Oklahoma LLC and West Shore LLC; (ii) the Operating Company owns 50% of the issued and outstanding membership interests in each of Pioneer LLC and Starfish LLC; (iii) Basin LLC owns 100% of the issued and outstanding membership interest in Matrex LLC; (iv) Oklahoma LLC owns 40% of the issued and outstanding membership interests in Centrahoma LLC and 100% of the issued and outstanding membership interests in McAlester LLC; and (v) Liberty LLC owns 60% of the issued and outstanding membership interests in Liberty Joint Venture  (the entities set forth in clauses (i) through (v) of this paragraph (u) are referred to herein collectively as the “LLC Operating Subs”); such membership interests have been duly authorized and validly issued in accordance with the limited liability company

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agreements of the LLC Operating Subs (as the same may be amended or restated at or prior to the Closing Date, the “Operating Subs LLC Agreements”) and are fully paid (to the extent required under the respective Operating Subs LLC Agreements) and nonassessable (except as such nonassessability may be affected by Sections 18-607 and 18-804 of the Delaware LLC Act, Sections 2033 and 2035 of the Oklahoma LLC Act, Article 5.09 of the Texas LLC Act, Section 101.206 of the TBOC or Section  ###-###-#### of the Michigan LLC Act); and each of the Operating Company, Basin LLC, Oklahoma LLC or Liberty LLC, as the case may be, owns such membership interests free and clear of all liens, encumbrances, security interests, equities, charges or claims, except as may arise under the Credit Agreement or as described in the Pricing Disclosure Package and the Prospectus.

(v)           Ownership of Interests in Bright Star and Wirth Gathering.  Pinnacle LLC owns 90% of the issued and outstanding general partnership interests in Bright Star and McAlester LLC owns 50% of the issued and outstanding general partnership interests in Wirth Gathering; such general partnership interests have been duly authorized and validly issued in accordance with the respective partnership agreements of Bright Star and Wirth Gathering (as the same have been and may be amended or restated at or prior to the applicable Closing Date, the “Operating Subs Partnership Agreements”); and each of Pinnacle LLC and McAlester LLC owns such general partnership interests free and clear of all liens, encumbrances, security interests, equities, charges or claims, except for such liens, encumbrances, security interests, equities, charges and claims arising under the Credit Agreement or described in the Pricing Disclosure Package and the Prospectus.

(w)          No Other Subsidiaries.  Other than (i) the Partnership’s (A) ownership of the capital stock of MarkWest Hydrocarbon, (B) membership interest in MarkWest Energy GP and the Operating Company and (C) ownership of 100% of the issued and outstanding shares of capital stock in MarkWest Energy Finance Corporation, a Delaware corporation, (ii) MarkWest Hydrocarbon’s membership interest in MarkWest Energy GP and Mason LLC, (iii) the Operating Company’s ownership of the membership interests in the LLC Operating Subs, as described in Section 1(u), (iv) Basin LLC’s membership interest in Matrex LLC, (v) Oklahoma LLC’s membership interests in Centrahoma LLC and McAlester LLC, (vi) Liberty LLC’s membership interest in Liberty Joint Venture, (vii) Pinnacle LLC’s ownership of the general partnership interests in Bright Star, (viii) McAlester LLC’s ownership of the general partnership interests in Wirth Gathering and (ix) Starfish’s ownership of 100% of the issued and outstanding membership interests in West Cameron Dehydration Company, LLC, a Delaware limited liability company, Stingray Pipeline Company, L.L.C., a Delaware limited liability company, and Triton Gathering, LLC, a Delaware limited liability company, none of the MarkWest Entities owns, and at the applicable Closing Date, none will own, directly or indirectly, any equity or long-term debt securities of any corporation, partnership, limited liability company, joint venture, association or other entity.

(x)            Preemptive Rights.  Except as may arise under the Credit Agreement or as described in the Pricing Disclosure Package and the Prospectus or in the partnership or limited liability company agreements of each MarkWest Entity that is a partnership or a limited liability company (the “Organizational Agreements”) or the certificate of

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formation or conversion, certificate or articles of incorporation, bylaws or other constituent document (collectively, the “Organizational Documents”) of the MarkWest Entities, there are no preemptive rights or other rights to subscribe for or to purchase, nor any restriction upon the voting or transfer of, any shares of capital stock of or partnership or membership interests in any of the MarkWest Entities.  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, other than as have been waived or deemed waived.  Except as described in the Pricing Disclosure Package and the Prospectus, there are no outstanding options or warrants to purchase any shares of capital stock of or partnership or member interests in any MarkWest Entity.

(y)           Authority and Authorization.  The Partnership has all requisite power and authority to issue, sell and deliver the Units to be sold by it hereunder, in accordance with and upon the terms and conditions set forth in this Agreement, the Partnership Agreement and the Registration Statement, the Pricing Disclosure Package and the Prospectus.  At the applicable Closing Date, all corporate, partnership and limited liability company action, as the case may be, required to be taken by the MarkWest Entities or any of their stockholders, partners or members for the authorization, issuance, sale and delivery of the Units to be sold by the Partnership hereunder and the consummation of the transactions contemplated by this Agreement, shall have been duly and validly taken.

(z)            Underwriting Agreement.  This Agreement has been duly authorized, validly executed and delivered by each of the MarkWest Parties, and constitutes the valid and legally binding agreement of each of the MarkWest Parties.

(aa)         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 terms; provided that, the enforceability thereof may be limited by (A) 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 (B) public policy, applicable law relating to fiduciary duties and indemnification and an implied covenant of good faith and fair dealing.

(bb)         No Conflicts.  None of the offering, issuance and sale by the Partnership of the Units to be sold by it hereunder, the execution, delivery and performance of this Agreement by the MarkWest Parties, or the consummation by the MarkWest Parties of the transactions contemplated hereby (i) conflicts or will conflict with or constitutes or will constitute a violation of the Organizational Agreements or the Organizational Documents, as applicable, of the MarkWest Entities, (ii) conflicts or will conflict with or constitutes or will constitute a breach or violation of, or a default (or an event which, 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 MarkWest Entities is a party or by which any of them or any of their respective properties may be bound, (iii) violates or will violate any statute, law or regulation or any

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order, judgment, decree or injunction of any court or governmental agency or body directed to any of the MarkWest Entities or any of their properties in a proceeding to which any of them or their property is a party or (iv) results or will result in the creation or imposition of any lien, charge or encumbrance upon any property or assets of any of the MarkWest Entities, which conflicts, breaches, violations, defaults, liens, changes or encumbrances, in the case of clauses (ii), (iii) or (iv), would, individually or in the aggregate, have a Material Adverse Effect.

(cc)         No Consents Required.  No permit, consent, approval, authorization, order, registration, filing or qualification (“consent”) of or with any court, governmental agency or body having jurisdiction over the MarkWest Entities or any of their respective properties is required for the offering, issuance and sale by the Partnership of the Units to be sold by it hereunder in connection with the execution, delivery and performance of this Agreement by the MarkWest Parties or the consummation by the MarkWest Parties of the transactions contemplated by this Agreement, except (i) for such consents required under the Securities Act, the Exchange Act and state securities or “Blue Sky” laws, (ii) for such consents that have been, or prior to the applicable Closing Date will be, obtained and (iii) for such consents that, if not obtained, would not, individually or in the aggregate, have a Material Adverse Effect..

(dd)         No Violation or Default.  None of the MarkWest Entities is in (i) violation of its Organizational Agreement or Organizational Document, as applicable, (ii) violation of any law, statute, ordinance, administrative or governmental rule or regulation applicable to it or of any order, judgment, decree or injunction of any court or governmental agency or body having jurisdiction over it or (iii) breach, default (or an event which, with notice or lapse of time or both, would constitute such a default) or violation in the performance of any obligation, agreement or condition contained in any bond, debenture, note or any other evidence of indebtedness or in any agreement, indenture, lease or other instrument to which it is a party or by which it or any of its properties may be bound, which breach, default or violation in the case of clause (ii) or (iii) would, if continued, have a Material Adverse Effect or could materially impair the ability of any of the MarkWest Parties to perform their respective obligations under this Agreement.  To the knowledge of the MarkWest Parties, no third party to any indenture, mortgage, deed of trust, loan agreement or other agreement or instrument to which any of the MarkWest Entities is a party or by which any of them is bound or to which any of their properties is subject, is in default under any such agreement, which breach, default or violation would, if continued, have a Material Adverse Effect.

(ee)         Conformity of Units.  The Units, when issued and delivered against payment therefor as provided herein, will conform in all material respects to the descriptions thereof contained in the Pricing Disclosure Package and the Prospectus.

(ff)           Independent Registered Public Accounting Firm.  Deloitte & Touche LLP, which has certified certain financial statements of the Partnership and MarkWest Hydrocarbon and delivered its opinion with respect to certain audited financial statements and schedules included or incorporated by reference in the Registration Statement, the most recent Preliminary Prospectus and the Prospectus (or any amendment or supplement

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thereto), is an independent registered public accounting firm with respect to the Partnership and MarkWest Hydrocarbon within the meaning of the Securities Act and the Rules and Regulations and the rules of the Public Company Accounting Oversight Board.

(gg)         Financial Statements.  At March 31, 2009, the Partnership would have had, on an actual, pro forma and pro forma as adjusted basis as indicated in the most recent Preliminary Prospectus and the Prospectus (and any amendment or supplement thereto), a capitalization as set forth therein.  The historical financial statements (including the related notes and supporting schedules) included in the Registration Statement, the most recent Preliminary Prospectus and the Prospectus (and any amendment or supplement thereto) present fairly in all material respects the financial position, results of operations and cash flows of the entities purported to be shown thereby on the basis stated therein at the respective dates or for the respective periods to which they apply and have been prepared in accordance with accounting principles generally accepted in the United States consistently applied throughout the periods involved, except to the extent disclosed therein.  The summary historical financial information set forth in the most recent Preliminary Prospectus and the Prospectus (and any amendment or supplement thereto) under the caption “Summary Historical Condensed Combined Financial and Operating Data” is accurately presented in all material respects and prepared on a basis consistent with the audited and unaudited historical consolidated financial statements from which it has been derived.  Each “forward-looking statement” included or incorporated by reference in the Registration Statement, the most recent Preliminary Prospectus and the Prospectus (and any amendment or supplement thereto), if any, has been made or reaffirmed with a reasonable basis and in good faith.

(hh)         Statistical and Market Data.  Nothing has come to the attention of the MarkWest Parties that has caused the MarkWest Parties to believe that the statistical and market-related data included or incorporated by reference in the most recent Preliminary Prospectus and the Prospectus are not based on or derived from sources that are reliable and accurate in all material respects.

(ii)           Certain Relationships.  Except as described in the Pricing Disclosure Package and the Prospectus, no relationship, direct or indirect, exists between or among the Partnership, on the one hand, and the directors, officers, unitholders, customers or suppliers of any of the MarkWest Entities, on the other hand, that is required to be described in the Pricing Disclosure Package or the Prospectus that is not so described.

(jj)           No Material Adverse Change.  Except as disclosed in the Pricing Disclosure Package and the Prospectus, none of the MarkWest Entities has sustained since the date of the latest audited financial statements included in the Registration Statement, the most recent Preliminary Prospectus and the Prospectus any material loss or interference with its business from fire, explosion, flood or other calamity, whether or not covered by insurance, or from any labor dispute or court or governmental action, investigation, order or decree, otherwise than as set forth or contemplated in the Pricing Disclosure Package and the Prospectus.  Except as disclosed in the Registration Statement, the Pricing Disclosure Package and the Prospectus (or any amendment or

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supplement thereto or any document filed with the Commission after the date hereof and incorporated by reference therein), subsequent to the respective dates as of which such information is given in the Registration Statement, the Pricing Disclosure Package and the Prospectus (or any amendment or supplement thereto or any document filed with the Commission after the date hereof and incorporated by reference therein), (i) none of the MarkWest Entities has incurred any liability or obligation, indirect, direct or contingent, or entered into any transactions, not in the ordinary course of business, that, individually or in the aggregate, is material to the MarkWest Entities, taken as a whole, (ii) there has not been any material change in the capitalization, or material increase in the short-term debt or long-term debt, of the MarkWest Entities, taken as a whole and (iii) there has not been any material adverse change, or any development involving or that may reasonably be expected to involve, individually or in the aggregate, a prospective material adverse change in or affecting the business, prospects, properties, management, condition (financial or other), partners’ capital, net worth or results of operations of the MarkWest Entities, taken as a whole.

(kk)         Legal Proceedings; Filing of Exhibits.  There are no legal or governmental proceedings pending or, to the knowledge of the MarkWest Parties, threatened against any of the MarkWest Entities, or to which any of the MarkWest Entities is a party, or to which any of their respective properties is subject, that are required to be described in the Registration Statement, the Pricing Disclosure Package or the Prospectus but 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, the Pricing Disclosure Package or the Prospectus or to be filed as exhibits to the Registration Statement that are not described or filed as required by the Rules and Regulations.

(ll)           Title to Properties.  The MarkWest Entities have good and indefeasible title to all real property and good title to all personal property described in the Pricing Disclosure Package and the Prospectus as owned by the MarkWest Entities, free and clear of all liens, claims, security interests, or other encumbrances, except such as (i) do not materially affect the value of such property and do not materially interfere with the use made and proposed to be made of such property by the MarkWest Entities, (ii) could not reasonably be expected to have a Material Adverse Effect or (iii) are described, and subject to the limitations contained, in the Pricing Disclosure Package and the Prospectus.

(mm)       Rights-of-Way.  Each of the MarkWest Entities has such consents, easements, rights-of-way, permits or licenses from each person (collectively, “rights-of-way”) as are necessary to conduct its business in the manner described, and subject to the limitations contained, in the Pricing Disclosure Package and the 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, individually or in the aggregate, a Material Adverse Effect; other than as set forth, and subject to the limitations contained, in the Pricing Disclosure Package and the Prospectus, each of the MarkWest Entities has fulfilled and performed all its 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

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and impairments that would not have a Material Adverse Effect; and, except as described in the Pricing Disclosure Package and the Prospectus, none of such rights-of-way contains any restriction that is materially burdensome to the MarkWest Entities, taken as a whole.

(nn)         Permits.  Each of the MarkWest Entities has, or at the applicable Closing Date will have, such permits, consents, licenses, franchises, certificates and authorizations issued by, and have made all declarations and filings with, the appropriate federal, state, local or foreign governmental or regulatory authorities (“permits”) as are necessary to own its properties and to conduct its business in the manner described in the Registration Statement, the Pricing Disclosure Package and the Prospectus, subject to such qualifications as may be set forth in the Registration Statement, the Pricing Disclosure Package and the Prospectus and except for such permits that, if not obtained, would not, individually or in the aggregate, have a Material Adverse Effect and except for any permits that are of a routine or administrative nature that are expected in the reasonable judgment of MarkWest Energy GP to be obtained, as necessary, in the ordinary course of business subsequent to the date hereof; each of the MarkWest Entities has fulfilled and performed all its material obligations with respect to such permits that are due to have been fulfilled and performed by such date in the manner described, and subject to the limitations contained, in the Registration Statement, the Pricing Disclosure Package and the Prospectus and no event has occurred that allows, or after notice or lapse of time would allow, revocation or termination thereof or results in any impairment of the rights of the holder of any such permit, except for such revocations, terminations and impairments that would not, individually or in the aggregate, have a Material Adverse Effect; and, except as described in the Registration Statement, the Pricing Disclosure Package and the Prospectus, none of such permits contains any restriction that is materially burdensome to the MarkWest Entities, taken as a whole.

(oo)         Books and Records; Accounting Controls.  Except as described in the Pricing Disclosure Package and the Prospectus, the Partnership (i) makes and keeps books, records and accounts, which, in reasonable detail, accurately and fairly reflect transactions and dispositions of assets and (ii) maintains effective internal control over financial reporting as defined in Rule 13a-15 under the Exchange Act and a system of internal accounting controls sufficient to provide reasonable assurances that (A) transactions are executed in accordance with management’s general or specific authorization; (B) transactions are recorded as necessary to permit preparation of financial statements in conformity with accounting principles generally accepted in the United States and to maintain accountability for its assets; (C) access to assets is permitted only in accordance with management’s general or specific authorization; and (D) the recorded accountability for assets is compared with existing assets at reasonable intervals and appropriate action is taken with respect to any differences.

(pp)         Disclosure Controls and Procedures.  (i) The Partnership has established and maintains disclosure controls and procedures (as such term is defined in Rule 13a-15 under the Exchange Act), (ii) such disclosure controls and procedures are designed to ensure that the 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

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communicated to management of the Partnership including its respective principal executive officers and principal financial officers, as appropriate, to allow such officers to make timely decisions regarding required disclosure and (iii) except as described in the Pricing Disclosure Package and the Prospectus, such disclosure controls and procedures are effective in all material respects to perform the functions for which they were established.

(qq)         No Adverse Changes in Internal Controls.  Since the date of the most recent balance sheet of the Partnership and its consolidated subsidiaries audited by Deloitte & Touche LLP, the Partnership has not been advised of (i) any significant deficiencies in the design or operation of internal controls that could adversely affect the Partnership’s ability to record, process, summarize and report financial data, or any material weaknesses in internal controls except as described in the Pricing Disclosure Package and the Prospectus or (ii) any fraud, whether or not material, that involves management or other employees who have a significant role in the Partnership’s internal controls.

(rr)           Sarbanes-Oxley Act of 2002.  There is and has been no failure on the part of the Partnership or any of the directors or officers of MarkWest Energy GP, in their capacities as such, to comply in all material respects with the provisions of the Sarbanes-Oxley Act of 2002 and the rules and regulations promulgated in connection therewith.

(ss)         Tax Returns.  Each of the MarkWest Entities has filed (or has obtained extensions with respect to) all material federal, state and foreign income and franchise tax returns required to be filed through the date hereof, which returns are complete and correct in all material respects, and has timely paid all taxes shown to be due, if any, pursuant to such returns, other than those (i) that are being contested in good faith and for which adequate reserves have been established in accordance with generally accepted accounting principles (ii) that, if not paid, would not have a Material Adverse Effect.

(tt)           Investment Company.  None of the MarkWest Entities is now, and after sale of the Units to be sold by the Partnership hereunder and application of the net proceeds from such sale as described in the Pricing Disclosure Package and the Prospectus under the caption “Use of Proceeds,” none of the MarkWest Entities will be, an “investment company” or a company “controlled by” an “investment company” within the meaning of the Investment Company Act of 1940, as amended.

(uu)         Environmental Compliance.  Except as described in the Pricing Disclosure Package and the Prospectus, the MarkWest Entities (i) are in compliance with any and all applicable federal, state and local laws and regulations relating to the protection of human health and safety and the environment or imposing liability or standards of conduct concerning any Hazardous Material (as hereinafter defined) (“Environmental Laws”), (ii) have received all permits required of them under applicable Environmental Laws to conduct their respective businesses, (iii) are in compliance with all terms and conditions of any such permit and (iv) do not have any liability in connection with the release into the environment of any Hazardous Materials (as defined below), except where such noncompliance with Environmental Laws, failure to receive required permits, or failure

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to comply with the terms and conditions of such permits or liability in connection with such releases would not, individually or in the aggregate, 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 other Environmental Law.

(vv)         No Labor Dispute.  No labor dispute with the employees of the MarkWest Entities exists or, to the knowledge of the MarkWest Parties, is imminent or threatened that is reasonably likely to result in a Material Adverse Effect.

(ww)       Insurance.  The MarkWest Entities maintain insurance covering the properties, operations, personnel and businesses of the MarkWest Entities against such losses and risks as are reasonably adequate to protect them and their businesses in a manner consistent with other businesses similarly situated.  None of the MarkWest Entities has received notice from any insurer or agent of such insurer that substantial capital improvements or other expenditures will have to be made in order to continue such insurance, and all such insurance is outstanding and duly in force on the date hereof and will be outstanding and duly in force on the applicable Closing Date.

(xx)          Litigation.  Except as described in the Pricing Disclosure Package and the 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 MarkWest Parties, threatened, to which any of the MarkWest Entities is or may be a party or to which the business or property of any of the MarkWest Entities is or may be subject, (ii) no statute, rule, regulation or order that has been enacted, adopted or issued by any governmental agency or, to the knowledge of the MarkWest Parties, proposed 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 MarkWest Entities is or may be subject, that, in the case of clauses (i), (ii) and (iii) above, is reasonably likely to (A) individually or in the aggregate 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.

(yy)         Distribution of Prospectus.  None of the MarkWest Entities has distributed and, prior to the later to occur of (i) any Closing Date and (ii) completion of the distribution of the Firm Units or Additional Units, as the case may be, will not distribute, any prospectus (as defined under the Securities Act) in connection with the offering and sale of the Units other than any Preliminary Prospectus, the Prospectus, any Issuer Free Writing Prospectus, subject to the conditions in Section 1(i) of this Agreement, or other materials, if any, permitted by the Securities Act, including Rule 134 of the Rules and Regulations.

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(zz)          NYSE Listing.  The Units have been approved for listing on the New York Stock Exchange, subject only to official notice of issuance.

(aaa)       No Stabilization.  None of the MarkWest Entities (i) has taken, and none of such persons shall take, directly or indirectly, any action designed to cause or result in, or which has constituted or which would reasonably be expected to constitute, the stabilization or manipulation of the price of the Common Units to facilitate the sale or resale of the Common Units in violation of any law, rule or regulation or (ii) since the initial filing of the Registration Statement, except as contemplated by this Agreement, (A) has sold, bid for, purchased or paid anyone any compensation for soliciting purchases of the Common Units or (B) has paid or agreed to pay to any person any compensation for soliciting another to purchase any other securities of the Partnership.

(bbb)      Significant Subsidiaries.  None of the MarkWest Parties has any subsidiaries, other than MarkWest Hydrocarbon, the Operating Company, Javelina Company LLC, Oklahoma LLC, East Texas LLC and Starfish LLC (collectively, the “Significant Subsidiaries”), which, individually or considered as a whole, would be deemed to be a significant subsidiary (as such term is defined in Rule 405 under the Securities Act).

(ccc)       No Unlawful Payments.  None of the MarkWest Entities or, to the knowledge of the MarkWest Entities, any director, officer, agent, employee or other person associated with or acting on behalf of any of the MarkWest Entities has (i) used any corporate funds for any unlawful contribution, gift, entertainment or other unlawful expense relating to political activity; (ii) made any direct or indirect unlawful payment to any foreign or domestic government official or employee from corporate funds; (iii) violated or is in violation of any provision of the Foreign Corrupt Practices Act of 1977; or (iv) made any bribe, rebate, payoff, influence payment, kickback or other unlawful payment.

(ddd)      No Restrictions on Subsidiaries.  No subsidiary of the Partnership is currently prohibited, directly or indirectly, under any agreement or other instrument to which it is a party or is subject, from paying any dividends to the Partnership, from making any other distribution on such subsidiary’s capital stock or equity interest, from repaying to the Partnership any loans or advances to such subsidiary from the Partnership or from transferring any of such subsidiary’s properties or assets to the Partnership or any other subsidiary of the Partnership.

Any certificate signed by any officer of any MarkWest Party and delivered to the Underwriter or to counsel for the Underwriter shall be deemed a representation and warranty by such MarkWest Party to the Underwriter as to the matters covered thereby.

2.     Agreements to Sell and Purchase.   On the basis of the representations and warranties contained in this Agreement, and subject to its terms and conditions, the Partnership hereby agrees to sell to the Underwriter, and the Underwriter agrees to purchase from the Partnership, the Firm Units at the purchase price set forth in Schedule I hereto (the “Purchase Price”).

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On the basis of the representations and warranties contained in this Agreement, and subject to its terms and conditions, the Partnership agrees to sell to the Underwriter the Additional Units, and the Underwriter shall have the right to purchase from the Partnership up to the number of Additional Units set forth in Schedule I hereto at the Purchase Price.  The Underwriter may exercise this right in whole or from time to time in part by giving written notice not later than 30 days after the date of this Agreement.  Any exercise notice shall specify the number of Additional Units to be purchased by the Underwriter and the date on which such Additional Units are to be purchased.  Each purchase date must be at least one business day after such written notice is given and may not be earlier than the Initial Closing Date (as such term is defined in Section 4 hereof) nor later than ten business days after the date of such notice.  Additional Units may be purchased as provided in Section 4 hereof solely for the purpose of covering over-allotments made in connection with the offering of the Firm Units.

3.     Public Offering.   The Partnership is advised by you that the Underwriter proposes to make a public offering of the Firm Units as soon as in your judgment is advisable.  The Partnership is further advised by you that the Units are to be offered to the public upon the terms set forth in the Prospectus.

4.     Payment and Delivery.   Payment for the Firm Units shall be made to the Partnership by wire transfer in immediately available funds to the account or accounts designated by the Partnership on the closing date and time set forth in Schedule I hereto, or at such other location and time on the same or such other date, not later than the fifth business day thereafter, as may be designated in writing by you.  The time and date of such payment are hereinafter referred to as the “Initial Closing Date”; each day, if any, that Additional Units are to be purchased is hereinafter referred to as an “Option Closing Date” and the Initial Closing Date and any Option Closing Date are each sometimes referred to as a “Closing Date.”

Payment for any Additional Units shall be made to the Partnership by wire transfer in immediately available funds to the account or accounts designated by the Partnership on the date specified in the corresponding notice described in Section 2 or at such other location and time on the same or on such other date, in any event not later than the tenth business day thereafter, as may be designated in writing by you.

The Firm Units and the Additional Units shall be registered in such names and in such denominations as you shall request in writing not later than one full business day prior to the Initial Closing Date or the applicable Option Closing Date, as the case may be, for the account of the Underwriter, with any transfer taxes payable in connection with the transfer of the Units to the Underwriter duly paid, against payment of the Purchase Price therefor.

5.     Covenants of the MarkWest Parties and the Underwriter.  (a) The MarkWest Parties covenant and agree with the Underwriter that:

(i)            The Partnership will timely transmit copies of the Prospectus, and any amendments or supplements thereto, to the Commission for filing pursuant to Rule 424(b) of the Rules and Regulations.

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(ii)           The Partnership will deliver to the Underwriter, and to counsel for the Underwriter (i) a signed copy of the Registration Statement as originally filed, including copies of exhibits thereto, and any amendments and supplements to the Registration Statement and (ii) a signed copy of each consent and certificate included in, or filed as an exhibit to, the Registration Statement as so amended or supplemented; the Partnership will deliver to the Underwriter as soon as practicable after the date of this Agreement as many copies of the Prospectus as the Underwriter may reasonably request for the purposes contemplated by the Securities Act; if there is a post-effective amendment to the Registration Statement that is not effective under the Securities Act, the Partnership will use its best efforts to cause such post-effective amendment to the Registration Statement to become effective as promptly as possible, and it will notify the Underwriter, promptly after it shall receive notice thereof, of the time when the post-effective amendment to the Registration Statement has become effective; the Partnership will promptly advise the Underwriter of any request of the Commission for amendment of the Registration Statement or for supplement to the Prospectus or for any additional information, and of the issuance by the Commission or any state or other jurisdiction or other regulatory body of any stop order under the Securities Act or other order suspending the effectiveness of the Registration Statement (as amended or supplemented) or preventing or suspending the use of the most recent Preliminary Prospectus or the Prospectus or suspending the qualification or registration of the Units for offering or sale in any jurisdiction, and of the institution or threat of any proceedings therefor, of which the Partnership shall have received notice or otherwise have knowledge prior to the completion of the distribution of the Units; and the Partnership will use its best efforts to prevent the issuance of any such stop order or other order and, if issued, to secure the prompt removal thereof.

(iii)          The Partnership will not file any amendment to the Registration Statement, supplement to the Prospectus (or any other prospectus relating to the Units filed pursuant to Rule 424(b) of the Rules and Regulations that differs from the Prospectus as filed pursuant to such Rule 424(b)), or any Preliminary Prospectus or Issuer Free Writing Prospectus of which the Underwriter shall not previously have been advised or to which the Underwriter shall have reasonably objected in writing after being so advised unless the Partnership shall have determined based upon the advice of counsel that such amendment, supplement or other filing is required by law; and the Partnership will promptly notify the Underwriter after it shall have received notice thereof of the time when any amendment to the Registration Statement becomes effective or when any supplement to the Prospectus has been filed.

(iv)          During the period when a prospectus relating to any of the Units is required to be delivered under the Securities Act by the Underwriter or any dealer, the Partnership will comply, at its own expense, with all requirements imposed by the Securities Act and the Rules and Regulations, so far as necessary, to permit the continuance of sales of or dealing in the Units during such period in accordance with the provisions hereof and as contemplated by the Prospectus.

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(v)           If, during the period when a prospectus relating to any of the Units is required to be delivered under the Securities Act by the Underwriter or any dealer, (A) any event relating to or affecting the Partnership or of which the Partnership shall be advised in writing by the Underwriter shall occur as a result of which, in the opinion of the Partnership or the counsel for the Underwriter, the Prospectus as then amended or supplemented would include any 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 or (B) it shall be necessary to amend or supplement the Registration Statement or the Prospectus to comply with the Securities Act, the Rules and Regulations, the Exchange Act or the rules and regulations thereunder, the Partnership will forthwith at its expense prepare and file with the Commission, and furnish to the Underwriter a reasonable number of copies of, such amendment or supplement or other filing that will correct such statement or omission or effect such compliance.

(vi)          During the period when a prospectus relating to any of the Units is required to be delivered under the Securities Act by the Underwriter or any dealer, the Partnership will furnish such proper information as may be lawfully required and otherwise cooperate with the Underwriter in qualifying the Units for offer and sale under the securities or “Blue Sky” laws of such jurisdictions as the Underwriter may reasonably designate and will file and make such statements or reports as are or may be reasonably necessary; provided, however, that the Partnership shall not be required to qualify as a foreign corporation, to qualify as a dealer in securities or to file a general consent to service of process under the laws of any jurisdiction.

(vii)         In accordance with Section 11(a) of the Securities Act and Rule 158 of the Rules and Regulations, the Partnership will make generally available to its security holders an earnings statement (which need not be audited) in reasonable detail covering the 12-month period beginning not later than the first day of the month next succeeding the month in which occurred the effective date (within the meaning of Rule 158) of the Registration Statement as soon as practicable after the end of such period.

(viii)        The Partnership will furnish or make available to its security holders annual reports containing financial statements audited by an independent registered public accounting firm and quarterly reports containing financial statements and financial information, which may be unaudited.  The Partnership will, for a period of two years from the latest Closing Date, furnish or make available to the Underwriter a copy of each annual report, quarterly report, current report and all other documents, reports and information furnished by the Partnership to holders of Units or filed with any securities exchange or market pursuant to the requirements of such exchange or market or with the Commission pursuant to the Securities Act or the Exchange Act.  The Partnership will deliver or make available to the Underwriter similar reports with respect to any significant subsidiaries, as that term is defined in the Rules and Regulations,

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which are not consolidated in the Partnership’s financial statements.  Any report, document or other information required to be furnished or made available under this paragraph (viii) shall be furnished as soon as practicable after such report, document or information becomes publicly available.

(ix)           The Partnership will not, for the period set forth in Schedule I hereto, directly or indirectly, (A) offer for sale, sell, pledge, or otherwise dispose of (or enter into any transaction or device that is designed to, or could be expected to, result in the disposition by any person at any time in the future of) any Common Units (including, without limitation, Common Units that may be deemed to be beneficially owned by the undersigned in accordance with the rules and regulations of the Commission and Common Units that may be issued upon exercise of any options or warrants) or securities convertible into or exercisable or exchangeable for Common Units, (2) enter into any swap or other derivatives transaction that transfers to another, in whole or in part, any of the economic benefits or risks of ownership of Common Units, whether any such transaction described in clause (1) or (2) above is to be settled by delivery of Common Units or other securities, in cash or otherwise, (3) make any demand for or exercise any right or cause to be filed a registration statement, including any amendments thereto, with respect to the registration of any Common Units or securities convertible into or exercisable or exchangeable for Common Units or any other securities of the Partnership (other than (a) a registration statement on Form S-8 in connection with employee benefit plans, qualified stock option plans or other employee compensation plans existing on the date hereof or (b) any registration statement or amendment thereto filed in connection with the satisfaction of registration rights obligations relating to the Partnership’s 6.875% Senior Notes due 2014) or (4) publicly disclose the intention to do any of the foregoing, in each case without the prior written consent of the Underwriter; and the Partnership will cause each of the directors and executive officers of MarkWest Energy GP to furnish to the Underwriter, prior to the Initial Closing Date, a letter or letters, substantially in the form of Schedule II hereto.