intention to do any of the foregoing, in each case without the prior written consent of the Representatives.  The Partnership will cause each of the directors and executive officers of MarkWest Energy GP to furnish to the Representatives, prior to the Initial Closing Date, a letter or letters, substantially in the form of Schedule III hereto, and the Selling Unitholder will furnish to the Representatives, prior to the Initial Closing Date, a letter substantially in the form of Schedule III hereto.

(x)            The Partnership will apply the proceeds from the sale of the Units as set forth in the description under “Use of Proceeds” in the most recent Preliminary Prospectus and the Prospectus.

(xi)           The Partnership will promptly provide the Representatives with copies of all correspondence to and from, and all documents issued to and by, the Commission in connection with the registration of the Units under the Securities Act.

(xii)          The Partnership will use its best efforts to obtain approval for, and maintain the listing of the Units on, the New York Stock Exchange.

(xiii)         The Partnership has complied and will comply with the requirements of Rule 433 under the Securities Act applicable to any Issuer Free Writing Prospectus, including timely filing with the Commission or retention where required and legending.  The Partnership represents that it has satisfied and agrees that it will satisfy the conditions under Rule 433 under the Securities Act to avoid a requirement to file with the Commission any electronic road show.  The Partnership agrees that if at any time following issuance of an Issuer Free Writing Prospectus any event occurred or occurs as a result of which such Issuer Free Writing Prospectus would conflict with the information in the Registration Statements, the most recent Preliminary Prospectus or the Prospectus or would include an untrue statement of a material fact or omit to state any material fact necessary in order to make the statements therein, in light of the circumstances then prevailing, not misleading, the Partnership will give prompt notice thereof to the Representatives and, if requested by the Representatives, will prepare and furnish without charge to each Underwriter an Issuer Free Writing Prospectus or other document which will correct such conflict, statement or omission; provided, however, that this representation and warranty shall not apply to any statements or omissions in an Issuer Free Writing Prospectus made in reliance upon and in conformity with information furnished in writing to the Partnership through the Representatives by or on behalf of any Underwriter specifically for inclusion therein.

(xiv)        The Partnership will prepare a final term sheet relating to the offering of the Units, containing only information that describes the final terms of the offering in a form consented to by the Representatives, and to file such final term sheet within the period required by Rule 433(d)(5)(ii) under the Securities Act following the date the final terms have been established for the offering of the Units.

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(b)           The Selling Unitholder covenants and agrees with each Underwriter that:

(i)            If, during the period when a prospectus relating to any of the Units is required to be delivered under the Securities Act by any Underwriter or any dealer, the Selling Unitholder becomes aware that any information regarding it in the Selling Unitholder Registration Statement, the Pricing Disclosure Package or the Prospectus includes an untrue statement of a material fact or omits 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, the Selling Unitholder will promptly notify the Partnership and the Representatives of such information.

(ii)           The Selling Unitholder has not, prior to the execution of this Agreement, distributed any “prospectus” (within the meaning of the Securities Act) or offering material in connection with the offering or sale of the Units other than the Registration Statements and the Pricing Disclosure Package, and will not, at any time on or after the execution of this Agreement, distribute any “prospectus” (within the meaning of the Securities Act) or offering material in connection with the offering or sale of the Units other than the Pricing Disclosure Package and the then most recent Preliminary Prospectus.

(c)          The Underwriters severally covenant and agree with the MarkWest Parties that:

(i)            Each Underwriter represents and agrees that it has not made and, without the prior consent of the Partnership and the Representatives, it will not make, any offer relating to the Units that would constitute an Issuer Free Writing Prospectus.  Any such Issuer Free Writing Prospectus the use of which has been consented to by the Partnership and the Representatives is listed on Schedule II hereto or constitutes a road show that is an Issuer Free Writing Prospectus under Rule 433 of the Rules and Regulations.

(ii)           Each Underwriter severally agrees that such Underwriter shall not include any “issuer information” (as defined in Rule 433) in any “free writing prospectus” (as defined in Rule 405) used or referred to by the Underwriters without the prior consent of the Partnership (any such issuer information with respect to whose use the Partnership has given its consent, “Permitted Issuer Information”); provided that (i) no such consent shall be required with respect to any such issuer information contained in any document filed by the Partnership with the Commission prior to the use of such free writing prospectus and (ii) “issuer information,” as used in this Section 6(c)(ii), shall not be deemed to include information prepared by or on behalf of such Underwriter on the basis of or derived from issuer information.

7.             Expenses.  Whether or not the transactions contemplated in this Agreement are consummated or this Agreement is terminated, the MarkWest Parties agree to pay or cause to be paid all expenses incident to the performance of its obligations under this Agreement,

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including: (a) the fees, disbursements and expenses of the Partnership’s counsel, the Partnership’s accountants and the Selling Unitholder’s counsel in connection with the registration and delivery of the Units under the Securities Act and all other fees or expenses in connection with the preparation and filing of the Registration Statements (including any exhibits thereto), any Preliminary Prospectus, the Prospectus, any Issuer Free Writing Prospectus and any amendment or supplement thereto, including the filing fees payable to the Commission relating to the Units (within the time required by Rule 456(b)(1), if applicable), all printing costs associated therewith, and the mailing and delivering of copies thereof to the Underwriters and dealers, (b) all costs and expenses related to the transfer and delivery of the Units to the Underwriters, including any transfer or other taxes payable thereon, (c) the cost of printing or producing any Blue Sky Memorandum and all expenses in connection with the qualification of the Units for offer and sale under state securities laws as provided in Section 6(a)(vi) hereof, including filing fees and the reasonable fees and disbursements of counsel for the Underwriters in connection with such qualification and in connection with the Blue Sky Memorandum, (d)  all costs and expenses incident to listing the Units on the New York Stock Exchange or any other national securities exchanges or foreign stock exchanges, (e) the costs and charges of any transfer agent, registrar or depositary, other than those to be paid by the Selling Unitholder in accordance with this Agreement, (f) the costs and expenses of the Partnership relating to investor presentations on any “road show” undertaken in connection with the marketing of the offering of the Units, including without limitation expenses associated with the preparation or dissemination of any electronic road show, 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 and lodging expenses of the representatives and officers of the MarkWest Entities and any such consultants, and the cost of any aircraft chartered in connection with the road show and (g) all other costs and expenses incident to the performance of the obligations of the MarkWest Parties hereunder for which provision is not otherwise made in this Section 7.  It is understood, however, that except as provided in this Section 7 or in Section 11, the Underwriters will pay all of their costs and expenses, including fees and disbursements of their counsel, transfer taxes payable on resale of any of the Units by it and any advertising expenses connected with any offers it may make. The provisions of this Section 7 shall not supersede or otherwise affect any agreement that the Partnership and the Selling Unitholder may otherwise have for the allocation of such expenses among themselves.

8.             Conditions to the Underwriters’ Obligations.  The respective obligations of the Underwriters hereunder are subject to the accuracy, when made and on each Closing Date, of the representations and warranties of the MarkWest Parties and the Selling Unitholder contained herein, to the performance by the MarkWest Parties and the Selling Unitholder of their obligations hereunder, and to each of the following additional terms and conditions:

(a)           All filings required by Rule 424 and Rule 430B of the Rules and Regulations shall have been made.  All material required to be filed by the Partnership pursuant to Rule 433(d) under the Securities Act shall have been filed with the Commission within the applicable time period prescribed for such filing by Rule 433 under the Securities Act.  No stop order (i) suspending the effectiveness of a Registration Statement or (ii) suspending or preventing the use of the most recent Preliminary Prospectus, the Prospectus or any Issuer Free Writing Prospectus shall have been issued and no proceeding for that purpose shall have been initiated or, to the knowledge of the

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Partnership or any Underwriter, threatened or contemplated by the Commission, and any request of the Commission for additional information (to be included in the Registration Statements or the Prospectus or otherwise) shall have been complied with to the reasonable satisfaction of the Underwriters.

(b)           No Underwriter shall have advised the Partnership on or prior to the applicable Closing Date that the Registration Statements, the most recent Preliminary Prospectus, the Prospectus or any Issuer Free Writing Prospectus or any amendment or supplement thereto contains an untrue statement of fact which, in the opinion of counsel to the Underwriters, is material, or omits to state a fact which, in the opinion of such counsel, is material and, in the case of the Registration Statements, is required to be stated therein, or, in all other cases, is necessary in order to make the statements therein, in the light of the circumstances under which they were made, not misleading.

(c)           On the applicable Closing Date, the Representatives shall have received the opinion of Vinson & Elkins L.L.P., counsel for the Partnership, addressed to the Representatives and dated the applicable Closing Date, to the effect that:

(i)            The Partnership has been duly formed and is validly existing in good standing as a limited partnership under the Delaware LP Act with all necessary partnership power and authority to own or lease its properties and to conduct its business in all material respects as described in the Registration Statements, 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 the jurisdictions set forth on Schedule IV to this Agreement.

(ii)           Each of the Subsidiaries, other than (A) Basin Pipeline L.L.C., a Michigan limited liability company, MarkWest McAlester, L.L.C., an Oklahoma limited liability company (“McAlester LLC”), MarkWest Michigan Pipeline Company, L.L.C., a Michigan limited liability company, Oklahoma LLC, Mason Pipeline Limited Liability Company, a Michigan limited liability company, Matrex L.L.C., a Michigan limited liability company, and Wirth Gathering (collectively, the “Excluded Subsidiaries”) and (B) Bright Star, has been duly formed and is validly existing in good standing as a corporation or limited liability company under the DGCL, the Delaware LLC Act, the Texas LLC Act or the TBOC, as applicable, with all necessary corporate or limited liability company power, as applicable, and authority to own or lease its properties and to conduct its business in all material respects as described in the Registration Statements, the Pricing Disclosure Package and the Prospectus.  Each such entity is duly registered or qualified as a foreign corporation or limited liability company, as applicable, for the transaction of business under the laws of the jurisdictions set forth on Schedule IV to this Agreement.

(iii)          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.

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(iv)          Bright Star has been duly formed and is validly existing in good standing as a general partnership under the laws of the State of Texas with all necessary partnership power and authority to own or lease its properties and to conduct its business in all material respects as described in the Registration Statements, the Pricing Disclosure Package and the Prospectus.

(v)           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).

(vi)          The Units to be issued and sold to the Underwriters by the Partnership pursuant to this Agreement and the limited partner interests represented thereby have been duly authorized by the Partnership and, when issued and delivered to the Underwriters against payment therefor in accordance with the terms of this Agreement, 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).

(vii)         The issued and outstanding limited partnership interests, membership interests, general partner interests and capital stock, as applicable, of each Subsidiary (other than the Excluded Subsidiaries) (i) has been duly authorized and validly issued in accordance with its Constituent Documents, (ii) is fully paid (to the extent required under the Constituent Agreements) and (iii) is non-assessable (except as such nonassessability may be affected by Sections 17-607 and 17-804 of the Delaware LP Act, Sections 18-607 and 18-804 of the Delaware LLC Act, Article 5.09 of the Texas LLC Act or Section 101.206 of the TBOC).

(viii)        The Partnership directly or indirectly owns the general partner interests, membership interests and capital stock, as applicable, in the Subsidiaries in all material respects as described in the Pricing Disclosure Package and in the Prospectus, free and clear of all Liens (except Liens created by or arising under the Credit Agreement, the DGCL or the Delaware LLC Act) (A) in respect of which a financing statement under the Uniform Commercial Code of the State of Delaware naming the Partnership or MarkWest Energy GP as debtor is on file in the office of the Secretary of State of the State of Delaware or (B) otherwise known to such counsel, without independent investigation.

(ix)           Except as described in the Pricing Disclosure Package and the Constituent Documents of the MarkWest Entities, as applicable, there are no preemptive rights or other rights to subscribe for or to purchase, nor any restrictions upon the voting or transfer of, any shares of capital stock of or partnership or membership interest in the Partnership, MarkWest Energy GP or

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any of the Significant Subsidiaries, in each case pursuant to the Constituent Documents, as applicable, or, to the knowledge of such counsel, any other agreement or instrument listed as an exhibit to the Registration Statements to which such entities are a party or by which any of them may be bound.  To the knowledge of such counsel and except as described in the Partnership Agreement, neither the filing of the Registration Statements 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.  To such counsel’s knowledge, except as described in the Pricing Disclosure Package and the Prospectus, there are no outstanding options or warrants to purchase any partnership or membership interests in the Partnership, the Operating Company or any of the Significant Subsidiaries.

(x)            Each of the Constituent Documents, as applicable, to which any of the MarkWest Entities (other than Bright Star, Centrahoma Processing LLC, a Delaware limited liability company, MarkWest Liberty Midstream & Resources, L.L.C., a Delaware limited liability company, MarkWest Pioneer, L.L.C., a Delaware limited liability company, Starfish LLC and Wirth Gathering) is a party has been duly authorized and validly executed and delivered by such entity that is a party thereto and, assuming due authorization, execution and delivery by each entity to such agreement other than such parties, each of the Constituent Documents (other than any Constituent Document governed by law other than Texas or Delaware law), as applicable, constitutes a valid and legally binding agreement of the MarkWest Entities that are parties thereto, enforceable against such entity in accordance with its respective terms, subject to (A) applicable 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 the fiduciary duties and indemnification and contribution and an implied covenant of good faith and fair dealing.

(xi)           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, the Registration Statements, the Pricing Disclosure Package and the Prospectus.

(xii)          This Agreement has been duly authorized and validly executed and delivered by each of the MarkWest Parties.

(xiii)         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, and the consummation by the MarkWest Parties of the transactions contemplated hereby will not (A) conflict with or result in a breach or violation of any of the terms or provisions of, or constitute a default under, or result in the creation or imposition of any lien, charge or encumbrance

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upon any of its property or assets pursuant to, any indenture, mortgage, deed of trust, loan agreement or other agreement or instrument filed or incorporated by reference as an exhibit to the Partnership’s annual report on Form 10-K for the year ended December 31, 2008, quarterly reports on Form 10-Q for the quarters ended March 31, 2009 and June 30, 2009 or any applicable current report on Form 8-K filed with the Commission beginning January 1, 2009, (B) result in any violation of the provisions of any Constituent Documents of the MarkWest Entities, as applicable, or (C) result in the violation of the DGCL, the Delaware LP Act, the Delaware LLC Act, the Texas Revised Partnership Act, the Texas LLC Act, the TBOC, other Texas law or federal law (provided that such counsel need not express any opinion with respect to compliance with any state securities or federal or state antifraud law) or, to such counsel’s knowledge, any judgment, order, rule or regulation of any court or arbitrator or governmental or regulatory authority known to such counsel, except, in the case of clauses (A) and (C) above, for any such conflict, breach or violation that would not, individually or in the aggregate, have a Material Adverse Effect.

(xiv)        The statements included in the Registration Statements and the most recent Preliminary Prospectus under the captions “Summary—The Offering,” “Description of Common Units,” “Cash Distribution Policy,” and “The Partnership Agreement” insofar as they purport to constitute summaries of the terms of the Common Units (including the Units) are accurate summaries of the terms thereof in all material respects.

(xv)         The statements included in the Registration Statements and the most recent Preliminary Prospectus under the captions “Cash Distribution Policy,” “The Partnership Agreement,” and “Investment in MarkWest Energy Partners by Employee Benefit Plans” insofar as they purport to constitute summaries of the terms of federal or Texas statutes, rules or regulations or the Delaware LP Act or the Delaware LLC Act,  any legal and governmental proceedings or any contracts and other documents, constitute accurate summaries of the terms of such statutes, rules and regulations, legal and governmental proceedings and contracts and other documents in all material respects.  The description of the federal statutes, rules and regulations set forth in the Partnership’s annual report on Form 10-K for the year ended December 31, 2008 under “Business—Regulatory Matters” and “Business—Environmental Matters” constitute accurate summaries of the terms of such statutes, rules and regulations in all material respects.

(xvi)        No permit, consent, approval, authorization, order, registration, filing or qualification (“consent”) under the DGCL, Delaware LP Act, the Delaware LLC Act, the Texas Revised Partnership Act, the Texas LLC Act, the TBOC or other Texas law or federal law is required for 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 by this Agreement, except for such consents required under the Securities Act, the

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Exchange Act or under state securities or “Blue Sky” laws, as to which such counsel need not express any opinion.

(xvii)       The Common Units and Class A Units conform in all material respects to the descriptions thereof in the most recent Preliminary Prospectus and the Prospectus.

(xviii)      The opinion of Vinson & Elkins L.L.P. that is filed as Exhibit 8.1 to the Registration Statements is confirmed and the Underwriters may rely upon such opinion as if it were addressed to it.

(xix)         The Partnership Registration Statement became effective under the Securities Act on April 29, 2009; to the knowledge of such counsel, no stop order suspending the effectiveness of the Partnership Registration Statement has been issued and no proceedings for that purpose have been instituted or threatened by the Commission; and any required filing of the Prospectus pursuant to Rule 424(b) has been made in the manner and within the time period required by such Rule.

(xx)          The Selling Unitholder Registration Statement became effective under the Securities Act on June 3, 2008; to the knowledge of such counsel, no stop order suspending the effectiveness of the Selling Unitholder Registration Statement has been issued and no proceedings for that purpose have been instituted or threatened by the Commission; and any required filing of the Prospectus pursuant to Rule 424(b) has been made in the manner and within the time period required by such Rule.

(xxi)         The Registration Statements and the Prospectus, when filed with the Commission and on the applicable Closing Date, were, on their face, appropriately responsive, in all material respects, to the requirements of the Securities Act, except that in each case such counsel need express no opinion with respect to the financial statements or other financial and statistical data contained in or omitted from the Registration Statements or the Prospectus.

(xxii)        To the knowledge of such counsel, (A) there are no legal or governmental proceedings pending or 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 Pricing Disclosure Package or the Prospectus but are not so described as required and (B) 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 Statements that are not described or filed as required by the Securities Act.

(xxiii)       None of the MarkWest Entities organized in Delaware or Texas is, nor after giving effect to the offering and sale of the Units and the application of the proceeds thereof as described in the Pricing Disclosure Package will any of

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such entities be, an “investment company” as such term is defined in the Investment Company Act of 1940, as amended.

In addition, such counsel shall state that they have participated in conferences with officers and other representatives of the MarkWest Entities, the independent public accountants of the Partnership, the Representatives and counsel for the Underwriters, at which the contents of the Registration Statements, the Pricing Disclosure Package and the Prospectus and related matters were discussed, and although such counsel has not independently verified, is not passing upon, and is not assuming any responsibility for the accuracy, completeness or fairness of the statements contained in, the Registration Statements, the Pricing Disclosure Package and the Prospectus (except to the extent specified in paragraphs (xxi), (xxii) and (xxiv) of the foregoing opinion), based on the foregoing, no facts have come to such counsel’s attention that lead such counsel to believe that:

(A)          the Registration Statements, as of the latest Effective Date, contained an untrue statement of a material fact or omitted to state a material fact required to be stated therein or necessary to make the statements therein not misleading,

(B)           the Pricing Disclosure Package, as of the Applicable Time, included an untrue statement of a material fact or omitted 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

(C)           the Prospectus, as of its date and on the applicable Closing Date, included or includes an untrue statement of a material fact or omitted or omits to state a material fact necessary to make the statements therein, in the light of the circumstances under which they were made, not misleading;

it being understood that such counsel expresses no statement or belief with respect to (i) the financial statements and related schedules, including the notes thereto and independent registered public accountants’ reports thereon, included or incorporated by reference in the Registration Statements, the Pricing Disclosure Package or the Prospectus, (ii) any other financial or statistical information included or incorporated by reference in or omitted from the Registration Statements, the Pricing Disclosure Package or the Prospectus and (iii) representations and warranties and other statements of fact included in the exhibits to the Registration Statements or any documents incorporated by reference.

In rendering such opinion, such counsel may (A) rely in respect of matters of fact upon representations of the MarkWest Parties set forth in this Agreement and upon certificates of officers and employees of the MarkWest Entities and upon information obtained from public officials, (B) assume that all documents submitted to them as originals are authentic, that all copies submitted to them conform to the originals thereof, and that the signatures on all documents examined by them are genuine, (C) state that their opinion is limited to federal laws, the DGCL, the Delaware LP Act, the Delaware LLC Act, the Texas Revised Partnership Act, the Texas LLC Act, the TBOC and other

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Texas law, (D) with respect to the opinions expressed in paragraphs (i) through (vi) above as to the due qualification or registration as a foreign corporation, limited partnership or limited liability company, as the case may be, of each of the MarkWest Entities, state that such opinions are based upon certificates of foreign qualification or registration provided by the Secretary of State of the states listed on Schedule IV hereto (each of which shall be dated as of a date not more than fourteen days prior to the applicable Closing Date and shall be provided to the Underwriters or their counsel), and (E) state that they express no opinion with respect to state or local taxes or tax statutes to which any of the limited partners of the Partnership or any of the other MarkWest Parties may be subject.

(d)           On the applicable Closing Date, the Representatives shall have received the opinion of Hall, Estill, Hardwick, Gable, Golden & Nelson, P.C., opining as to the law of Oklahoma, addressed to the Representatives and dated the applicable Closing Date, to the effect that:

(i)            McAlester LLC has been duly formed and is validly existing in good standing as a limited liability company under the Oklahoma LLC Act with all necessary limited liability company power and authority to own or lease its properties owned or leased at the applicable Closing Date and to conduct its business in all material respects as described in each of the Registration Statements, the Pricing Disclosure Package and the Prospectus.  McAlester LLC is not qualified or registered as a foreign limited liability company in any jurisdiction.

(ii)           Oklahoma LLC has been duly formed and is validly existing in good standing as a limited liability company under the Oklahoma LLC Act with all necessary limited liability company power and authority to own or lease its properties owned or leased at the applicable Closing Date and to conduct its business in all material respects as described in each of the Registration Statements, the Pricing Disclosure Package and the Prospectus.  Oklahoma LLC is duly qualified or registered as a foreign limited liability company for the transaction of business under the laws of Texas and no other jurisdiction.

(iii)          The Operating Company directly owns of record 100% of the issued and outstanding membership interests in Oklahoma LLC; the membership interests of Oklahoma LLC have been duly authorized and validly issued in accordance with its limited liability company agreement and are fully paid (to the extent required under such limited liability company agreement) and nonassessable; and the Operating Company owns such membership interests free and clear of all Liens (A) in respect of which a financing statement under the Uniform Commercial Code of the State of Oklahoma naming the Operating Company as debtor is on file in the office of the Secretary of State of the State of Oklahoma or (B) otherwise known to such counsel, without independent investigation, other than those (X) mentioned in the Registration Statements, the Pricing Disclosure Package or the Prospectus, (Y) created by or arising under the Oklahoma LLC Act or (Z) arising under the Credit Agreement.

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(iv)          Oklahoma LLC directly owns of record 100% of the issued and outstanding membership interests in McAlester LLC;  the membership interests of McAlester LLC have been duly authorized and validly issued in accordance with the limited liability company agreement of McAlester LLC and are fully paid (to the extent required under such limited liability company agreement) and nonassessable; and Oklahoma LLC owns such membership interests free and clear of all Liens (A) in respect of which a financing statement under the Uniform Commercial Code of the State of Oklahoma naming Oklahoma LLC as debtor is on file in the office of the Secretary of State of the State of Oklahoma or (B) otherwise known to such counsel, without independent investigation, other than those (X) mentioned in the Registration Statements, the Pricing Disclosure Package or the Prospectus, (Y) created by or arising under the Oklahoma LLC Act or (Z) arising under the Credit Agreement.

(v)           Oklahoma LLC directly owns certain of the membership interests in Centrahoma LLC free and clear of all Liens (A) in respect of which a financing statement under the Uniform Commercial Code of the State of Oklahoma naming Oklahoma LLC as debtor is on file in the office of the Secretary of State of the State of Oklahoma or (B) otherwise known to such counsel, without independent investigation, other than those (X) mentioned in the Registration Statements, the Pricing Disclosure Package or the Prospectus, (Y) created by or arising under the Oklahoma LLC Act or (Z) arising under the Credit Agreement.

(vi)          Except as described in the Pricing Disclosure Package and the Prospectus, there are no preemptive rights or other rights to subscribe for or to purchase, nor any restriction upon the voting or transfer of any membership interests in Oklahoma LLC or McAlester LLC, pursuant to the limited liability company agreement of Oklahoma LLC or McAlester LLC, as applicable (except for the restrictions as to the transfer of membership interests of Oklahoma LLC or McAlester LLC as set forth in the limited liability company agreements of Oklahoma LLC or McAlester LLC, as applicable), or any other agreement or instrument listed as an exhibit to the Registration Statements to which Oklahoma LLC or McAlester LLC is expressly by name identified thereon as a party.  To the knowledge of such counsel, except as described in the Pricing Disclosure Package and the Prospectus, there are no outstanding options or warrants to purchase any membership interests in Oklahoma LLC or McAlester LLC.

(vii)         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, and the consummation by the MarkWest Parties of the transactions contemplated hereby (A) constitutes or will constitute a violation of the limited liability company agreement of Oklahoma LLC or McAlester LLC, (B) violates or will violate Oklahoma law or (C) results or will result in the creation or imposition of any Lien upon the property or assets of Oklahoma LLC or McAlester LLC, which violations or Liens, in the case of clauses (B) or (C) would, individually or in the aggregate, have a Material Adverse Effect on the MarkWest Entities, taken as a whole.

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In rendering such opinions, such counsel shall state that it (A) relied in respect of matters of fact upon certificates of officers and employees of the MarkWest Entities and upon information obtained from public officials including, without limitation, certificates of foreign qualification for each of the MarkWest Entities provided by the Secretary of State of Texas dated as of a recent date with respect to the opinion in paragraphs (i) above and (ii) assumed that all documents submitted to such counsel as originals are authentic, and all copies submitted to such counsel conform to the originals thereof, and that the signatures on all documents examined by such counsel are genuine.

The opinions expressed by such counsel may be limited to the laws of the State of Oklahoma, excepting therefrom municipal and local ordinances and regulations.  Such counsel need not express an opinion with respect to (i) title to any of the real or personal property, (ii) the accuracy of descriptions or references to real or personal property, (iii) permits to own or operate any real or personal property or (iv) with respect to state or local taxes or tax statutes including, without limitation, those to which any of the limited partners of the MarkWest Parties may be subject.

In rendering such opinion, such counsel shall state that each of the MarkWest Parties, Vinson & Elkins L.L.P. and Baker Botts L.L.P. are hereby authorized to rely upon such opinion letter as if such opinion letter were addressed and delivered to each of them.

(e)           On the applicable Closing Date, the Representatives shall have received the opinion of Cooley Godward Kronish LLP, counsel for the Selling Unitholder, addressed to the Representatives and dated the applicable Closing Date, to the effect that:

(i)            This Agreement has been duly authorized, executed and delivered by or on behalf of the Selling Unitholder.

(ii)           The Selling Unitholder has all requisite corporate power and authority to enter into this Agreement, the Custody Agreement and the Power of Attorney and to sell, transfer and deliver the Units to be sold by the Selling Unitholder and a security entitlement in respect of such Units.

(iii)          The Power of Attorney and the Custody Agreement have been duly authorized, executed and delivered by the Selling Unitholder.

(iv)          The Power of Attorney and the Custody Agreement constitute valid and binding obligations of the Selling Unitholder, enforceable in accordance with their terms, except where (1) the enforceability thereof may be limited by bankruptcy, insolvency, reorganization, fraudulent conveyance, moratorium, debtor and creditor or other similar laws now or hereafter in effect relating to rights of creditors and other obligees generally, (2) the remedy of specific performance and other forms of equitable relief may be subject to certain equitable defenses and principles and to the discretion of the court before which the proceedings may be brought and (3) rights to indemnity, contribution, reimbursement and the like may be limited by applicable law and public policy.

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(v)           Upon payment for the Units to be sold by the Selling Unitholder pursuant to this Agreement, delivery of such Units, as directed by the Underwriters, to Cede or such other nominee as may be designated by DTC, by registration of such Units in the name of Cede or such other nominee as may be designated by DTC and the crediting of such Units on the books of DTC to the securities accounts of the Underwriters (assuming that neither DTC nor any such Underwriter has notice of any “adverse claim” (within the meaning of Section 8-105 of the UCC) with respect to such Units), DTC shall be a “protected purchaser” (within the meaning of Section 8-303 of the UCC) of such Units and each Underwriter will have acquired a “securities entitlement” (within the meaning of Section 8-102(a)(17) of the UCC) with respect to such Units, free of any “adverse claim” (within the meaning of Section 8-102 of the UCC) with respect thereto.  For purposes of this opinion, such counsel may assume that the Units are “uncertificated securities” (as defined under Section 8-102(a)(18) of the UCC) and such counsel may further assume that when such payment, delivery and crediting occur, (A) such Units will have been registered in the name of Cede or another nominee designated by DTC, in each case on the Partnership’s unit registry in accordance with the Partnership Agreement and applicable law, (B) DTC will be registered as a “clearing corporation” (within the meaning of Section 8-102 of the UCC) and (C) appropriate entries to the accounts of the several Underwriters on the records of DTC will have been made pursuant to the UCC and that such accounts are “securities accounts” (as defined in Section 8-501(a) of the UCC).

(vi)         The execution and delivery by the Selling Unitholder of this Agreement, the Custody Agreement and the Power of Attorney, and the performance by the Selling Unitholder of its obligations under this Agreement, the Custody Agreement and the Power of Attorney as of the date of this Agreement, will not: (a) require any consent, approval, license or exemption by, order or authorization of, or filing, recording or registration by such Selling Unitholder with, any Colorado, New York or federal governmental authority, except such as have been made or obtained under the Securities Act and such consents, approvals, licenses, authorizations, filings, recordings or registrations under the Exchange Act (as to which such counsel need not express any opinion), and applicable state securities laws (as to which such counsel need not express any opinion), (b) constitute a default under or a material breach of any of the terms, conditions or provisions of the articles of incorporation or bylaws of the Selling Unitholder and any agreement or other instrument binding upon the Selling Unitholder, or (c) constitute a material violation of the Colorado, New York or federal statute, rule or regulation by which the Selling Unitholder is bound that in our experience is typically applicable to transactions of the nature contemplated by this Agreement.

(f)            The Representatives shall have received on the applicable Closing Date, from Baker Botts L.L.P., counsel to the Underwriters, such opinion or opinions, dated the

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applicable Closing Date with respect to such matters as the Representatives may reasonably require; and the Partnership shall have furnished to such counsel such documents as they reasonably request for the purposes of enabling them to review or pass on the matters referred to in this Section 8 and in order to evidence the accuracy, completeness and satisfaction of the representations, warranties and conditions herein contained.

(g)           At the time of the execution of this Agreement, the Representatives shall have received from Deloitte & Touche LLP, independent registered public accountants, a letter or letters dated such date, in form and substance satisfactory to the Representatives, together with signed or reproduced copies of such letters for each of the other Underwriters, containing statements and information of the type ordinarily included in accountants’ “comfort letters” to underwriters with respect to the financial statements and certain financial information contained in the Registration Statements and the Prospectus; provided that the cut-off date for the procedures performed by such accountants and described in such letters shall be a date not more than five days prior to the date of such letter.

(h)           On each applicable Closing Date, the Representatives shall have received from Deloitte & Touche LLP a letter, dated as of such Closing Date, to the effect that they reaffirm the statements made in the letter furnished pursuant to paragraph (g) of this Section 8, except that the date referred to in the proviso in Section 8(g) hereof shall be a date not more than three business days prior to such Closing Date.

(i)            Except as set forth in the Registration Statements, the Pricing Disclosure Package and the Prospectus, (i) none of the MarkWest Entities shall have sustained since the date of the latest financial statements included in the Registration Statements, the most recent Preliminary Prospectus and the Prospectus any 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, order or decree; and (ii) subsequent to the respective dates as of which such information is given in the Registration Statement, the most recent Preliminary Prospectus and the Prospectus (or any amendment or supplement thereto), none of the MarkWest Entities shall have incurred any liability or obligation, direct or contingent, or entered into any transactions, and there shall not have been any change in the issued and outstanding equity interests or short-term or long-term debt of the MarkWest Entities or any change, or any development involving or which might reasonably be expected to involve a prospective change in the condition (financial or other), net worth, business, affairs, management, prospects, results of operations or cash flow of the Partnership or its subsidiaries, the effect of which, in any such case described in clause (i) or (ii), is in the judgment of the Representatives so material or adverse as to make it impracticable or inadvisable to proceed with the public offering or the delivery of the Units being delivered on such Closing Date on the terms and in the manner contemplated in the Pricing Disclosure Package and the Prospectus.

(j)            The Underwriters shall have received on the applicable Closing Date a certificate of the Partnership, signed on behalf of the Partnership by the Chief Executive

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Officer and the Chief Financial Officer of MarkWest Energy GP, dated the applicable Closing Date, to the effect that the signers of such certificate have carefully examined the Registration Statements, the Pricing Disclosure Package, the Prospectus and any amendment or supplement thereto, as well as each electronic road show used in connection with the offering of the Units, and this Agreement and that:

(i)            the representations and warranties of the MarkWest Parties in this Agreement are true and correct on and as of the applicable Closing Date with the same effect as if made on such Closing Date, and the Partnership has complied with all the agreements and satisfied all the conditions on its part to be performed or satisfied at or prior to such Closing Date;

(ii)           no stop order suspending the effectiveness of the Registration Statement or any notice objecting to its use has been issued and no proceedings for that purpose have been instituted or, to the MarkWest Parties’ knowledge, threatened;

(iii) since the date of the most recent financial statements included or incorporated by reference in the Registration Statements, the Pricing Disclosure Package and the Prospectus, there has been no Material Adverse Effect, except as set forth in or contemplated in the Pricing Disclosure Package and the Prospectus; and

(iv)          in their opinion, (1) the Registration Statements, as of the most recent Effective Date, (2) the Prospectus, as of the date of the Prospectus and as of the applicable Closing Date, and (3) the Pricing Disclosure Package, as of the Applicable Time, did not and do not contain any untrue statement of a material fact and did not and do not omit to state a material fact required to be stated therein or necessary to make the statements therein (except in the case of the Registration Statement, in the light of the circumstances under which they were made) not misleading.

(k)           The Underwriters shall have received on the Closing Date a certificate of the Selling Unitholder, signed on behalf of the Selling Unitholder by a duly authorized officer of the Selling Unitholder, dated the Closing Date, to the effect that the representations and warranties of the Selling Unitholder in this Agreement are true and correct on and as of the applicable Closing Date with the same effect as if made on such Closing Date, and the Selling Unitholder has complied with all the agreements and satisfied all the conditions on its part to be performed or satisfied at or prior to such Closing Date.

(l)            The MarkWest Parties shall not have failed, refused, or been unable, at or prior to the applicable Closing Date to have performed any agreement on their part to be performed or any of the conditions herein contained and required to be performed or satisfied by them at or prior to such Closing Date.

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(m)          The Partnership shall have furnished to the Representatives at the applicable Closing Date such further information, opinions, certificates, letters and documents as the Underwriters may have reasonably requested.

(n)           The New York Stock Exchange shall have approved the Units for listing, subject only to official notice of issuance.

(o)           There shall not have occurred any downgrading, nor shall any notice have been given of any intended or potential downgrading or of any review for a possible change that does not indicate the direction of the possible change, in the rating accorded any of the securities of the Partnership or any of its subsidiaries by any “nationally recognized statistical rating organization,” as such term is defined for purposes of Rule 436(g)(2) under the Securities Act

(p)           The Representatives shall have received duly and validly executed letter agreements referred to in Section 6(a)(ix) hereof.

All such opinions, certificates, letters and documents will be in compliance with the provisions hereof only if they are satisfactory in form and substance to the Representatives and to Baker Botts L.L.P., counsel for the Underwriters.  The Partnership will furnish the Representatives with such signed and conformed copies of such opinions, certificates, letters and documents as the Representatives may reasonably request.

If any of the conditions specified above in this Section 8 shall not have been satisfied at or prior to the applicable Closing Date or waived by the Representatives in writing, this Agreement may be terminated by the Representatives on notice to the Partnership.

9.             Indemnification and Contribution.

(a)           The MarkWest Parties, jointly and severally, agree to indemnify and hold harmless each Underwriter, each person, if any, who controls any Underwriter within the meaning of either Section 15 of the Securities Act or Section 20 of the Exchange Act and each affiliate of any Underwriter within the meaning of Rule 405 under the Securities Act from and against any and all losses, claims, damages and liabilities (including, without limitation, any legal or other expenses reasonably incurred in connection with defending or investigating any such action or claim) caused by any untrue statement or alleged untrue statement of a material fact contained in the Registration Statement or any amendment thereof, any Preliminary Prospectus, the Pricing Disclosure Package, any Issuer Free Writing Prospectus as defined in Rule 433(h) under the Securities Act, any Partnership information that the Partnership has filed, or is required to file, pursuant to Rule 433(d) under the Securities Act, or the Prospectus or any amendment or supplement thereto, or caused by any omission or alleged omission to state therein a material fact required to be stated therein or necessary to make the statements therein not misleading, except insofar as such losses, claims, damages or liabilities are caused by any such untrue statement or omission or alleged untrue statement or omission based upon information relating to any Underwriter furnished to the Partnership through the Representatives by or on behalf of any Underwriter expressly for use therein.

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(b)           The Selling Unitholder agrees to indemnify and hold harmless each Underwriter, each person, if any, who controls any Underwriter within the meaning of either Section 15 of the Securities Act or Section 20 of the Exchange Act and each affiliate of any Underwriter within the meaning of Rule 405 under the Securities Act from and against any and all losses, claims, damages and liabilities (including, without limitation, any legal or other expenses reasonably incurred in connection with defending or investigating any such action or claim) caused by any untrue statement or alleged untrue statement of a material fact contained in the Registration Statements or any amendment thereof, any preliminary prospectus, the Pricing Disclosure Package, any issuer free writing prospectus as defined in Rule 433(h) under the Securities Act, any Partnership information that the Partnership has filed, or is required to file, pursuant to Rule 433(d) under the Securities Act, or the Prospectus or any amendment or supplement thereto, or caused by any omission or alleged omission to state therein a material fact required to be stated therein or necessary in order to make the statements therein not misleading, but only with reference to information relating to the Selling Unitholder furnished in writing by or on behalf of the Selling Unitholder expressly for use in a Registration Statement, any preliminary prospectus, the Prospectus or any amendment or supplement thereto.  The liability of the Selling Unitholder contained in this paragraph 9(b) shall be limited to an amount equal to the aggregate public offering price of the Units sold by the Selling Unitholder under this Agreement.

(c)           Each Underwriter agrees, severally and not jointly, to indemnify and hold harmless the Selling Unitholder and each of the MarkWest Parties and each of their directors, each of their officers who sign a Registration Statement and each person, if any, who controls the MarkWest Parties within the meaning of either Section 15 of the Securities Act or Section 20 of the Exchange Act to the same extent as the foregoing indemnity from the Sellers to such Underwriter, but only with reference to information relating to such Underwriter furnished to the Partnership through the Representatives by or on behalf of any Underwriter expressly for use in a Registration Statement, any Preliminary Prospectus, the Pricing Disclosure Package, any Issuer Free Writing Prospectus or the Prospectus or any amendment or supplement thereto.

(d)           In case any proceeding (including any governmental investigation) shall be instituted involving any person in respect of which indemnity may be sought pursuant to Section 9(a), 9(b) or 9(c), such person (the “indemnified party”) shall promptly notify the person against whom such indemnity may be sought (the “indemnifying party”) in writing and the indemnifying party, upon request of the indemnified party, shall retain counsel reasonably satisfactory to the indemnified party to represent the indemnified party and any others the indemnifying party may designate in such proceeding and shall pay the fees and disbursements of such counsel related to such proceeding.  In any such proceeding, any indemnified party shall have the right to retain its own counsel, but the fees and expenses of such counsel shall be at the expense of such indemnified party unless (i) the indemnifying party and the indemnified party shall have mutually agreed to the retention of such counsel or (ii) the named parties to any such proceeding (including any impleaded parties) include both the indemnifying party and the indemnified party and representation of both parties by the same counsel would be inappropriate due to actual or potential differing interests between them. It is understood

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that the indemnifying party shall not, in respect of the legal expenses of any indemnified party in connection with any proceeding or related proceedings in the same jurisdiction, be liable for (A) the fees and expenses of more than one separate firm (in addition to any local counsel) for all Underwriters and all persons, if any, who control any Underwriter within the meaning of either Section 15 of the Securities Act or Section 20 of the Exchange Act or who are affiliates of any Underwriter within the meaning of Rule 405 under the Securities Act, (B) the fees and expenses of more than one separate firm (in addition to any local counsel) for the MarkWest Parties, their directors, their officers who sign the Registration Statement and each person, if any, who controls the MarkWest Parties within the meaning of either such Section; and (C) the fees and expenses of more than one separate firm (in addition to any local counsel) for the Selling Unitholder and all persons, if any, who control the Selling Unitholder within the meaning of either such Section, and that all such fees and expenses shall be reimbursed as they are incurred.  In the case of any such separate firm for the Underwriters and such control persons and affiliates of any Underwriters, such firm shall be designated in writing by the Representatives. In the case of any such separate firm for the MarkWest Parties, and such directors, officers and control persons of the MarkWest Parties, such firm shall be designated in writing by the Partnership. In the case of any such separate firm for the Selling Unitholder and such control persons of the Selling Unitholder, such firm shall be designated in writing by the persons named as attorneys-in-fact for the Selling Unitholder under the Power of Attorney. The indemnifying party shall not be liable for any settlement of any proceeding effected without its written consent (which consent shall not be unreasonably withheld), but if settled with such consent or if there be a final judgment for the plaintiff, the indemnifying party agrees to indemnify the indemnified party from and against any loss or liability by reason of such settlement or judgment.  Notwithstanding the foregoing sentence, if at any time an indemnified party shall have requested an indemnifying party to reimburse the indemnified party for fees and expenses of counsel as contemplated by the second and third sentences of this paragraph, the indemnifying party agrees that it shall be liable for any settlement of any proceeding effected without its written consent if (i) such settlement is entered into more than 30 days after receipt by such indemnifying party of the aforesaid request and (ii) such indemnifying party shall not have reimbursed the indemnified party in accordance with such request prior to the date of such settlement.  No indemnifying party shall, without the prior written consent of the indemnified party (which consent shall not be unreasonably withheld), 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.

(e)           To the extent the indemnification provided for in Section 9(a), 9(b) or 9(c) is unavailable to an indemnified party or insufficient in respect of any losses, claims, damages or liabilities referred to therein, then each indemnifying party under such paragraph, in lieu of indemnifying such indemnified party thereunder, shall contribute to the amount paid or payable by such indemnified party as a result of such losses, claims, damages or liabilities (i) in such proportion as is appropriate to reflect the relative benefits received by the indemnifying party or parties on the one hand and the

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indemnified party or parties on the other hand from the offering of the Units or (ii) if the allocation provided by clause 9(e)(i) above is not permitted by applicable law, in such proportion as is appropriate to reflect not only the relative benefits referred to in clause 9(e)(i) above but also the relative fault of the indemnifying party or parties on the one hand and of the indemnified party or parties on the other hand in connection with the statements or omissions that resulted in such losses, claims, damages or liabilities, as well as any other relevant equitable considerations.  The relative benefits received by the Sellers on the one hand and the Underwriters on the other hand in connection with the offering of the Units shall be deemed to be in the same respective proportions as the net proceeds from the offering of the Units (before deducting expenses) received by each Seller and the total underwriting discounts and commissions received by the Underwriters, in each case as set forth in the table on the cover of the Prospectus, bear to the aggregate public offering price of the Units.  The relative fault of the Sellers on the one hand and the Underwriters on the other hand shall be determined by reference to, among other things, whether the untrue or alleged untrue statement of a material fact or the omission or alleged omission to state a material fact relates to information supplied by the Sellers or the Underwriters and the parties’ relative intent, knowledge, access to information and opportunity to correct or prevent such statement or omission.  The Underwriters’ respective obligations to contribute pursuant to this Section 9 are several in proportion to the respective number of Units they have purchased hereunder, and not joint.  The liability of the Selling Unitholder under the contribution agreement contained in this paragraph shall be limited to an amount equal to the aggregate public offering price of the Units sold by the Selling Unitholder under this Agreement.

(f)            The Sellers and the Underwriters agree that it would not be just or 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 Section 9(e).  The amount paid or payable by an indemnified party as a result of the losses, claims, damages and liabilities referred to in Section 9(e) shall be deemed to include, subject to the limitations set forth above, any legal or other expenses reasonably incurred by such indemnified party in connection with investigating or defending any such action or claim.  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 it and distributed to the public were offered to the public exceeds the amount of any damages that such Underwriter has otherwise been required to pay by reason of such untrue or alleged untrue statement or omission or alleged omission.  No person guilty of fraudulent misrepresentation (within the meaning of Section 11(f) of the Securities Act) shall be entitled to contribution from any person who was not guilty of such fraudulent misrepresentation.  The remedies provided for in this Section 9 are not exclusive and shall not limit any rights or remedies which may otherwise be available to any indemnified party at law or in equity.

(g)           The indemnity and contribution provisions contained in this Section 9 and the representations, warranties and other statements of the MarkWest Parties and the Selling Unitholder contained in this Agreement shall remain operative and in full force and effect regardless of (i) any termination of this Agreement, (ii) any investigation made

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by or on behalf of any Underwriter, any person controlling any Underwriter or any affiliate of any Underwriter, by or on behalf of the Selling Unitholder or any person controlling the Selling Unitholder, or by or on behalf of the MarkWest Parties, each of their officers or directors or any person controlling the MarkWest Parties and (iii) acceptance of and payment for any of the Units.

10.   Termination.   The Underwriters may terminate this Agreement by notice given by the Representatives to the Partnership, if after the execution and delivery of this Agreement and prior to the applicable Closing Date (i) trading generally shall have been suspended or materially limited on, or by, the New York Stock Exchange or the NASDAQ Global Market, (ii) trading of any securities of the Partnership shall have been suspended on any exchange or in any over-the-counter market, (iii) a material disruption in securities settlement, payment or clearance services in the United States shall have occurred, (iv) any moratorium on commercial banking activities shall have been declared by Federal or New York State authorities or (v) there shall have occurred any outbreak or escalation of hostilities, or any change in financial markets or any calamity or crisis that, in the Representatives’ judgment, is material and adverse and which, singly or together with any other event specified in this clause (v), makes it, in the Representatives’ judgment, impracticable or inadvisable to proceed with the offer, sale or delivery of the Units on the terms and in the manner contemplated in the Pricing Disclosure Package or the Prospectus.

11.   Defaulting Underwriters.   This Agreement shall become effective upon the execution and delivery hereof by the parties hereto.

If, on the Closing Date or an Option Closing Date, as the case may be, any one or more of the Underwriters shall fail or refuse to purchase Units that it has or they have agreed to purchase hereunder on such date, and the aggregate number of Units that such defaulting Underwriter or Underwriters agreed but failed or refused to purchase is not more than one-tenth of the aggregate number of the Units to be purchased on such date, the other Underwriters shall be obligated severally in the proportions that the number of Firm Units set forth opposite their respective names in Schedule I bears to the aggregate number of Firm Units set forth opposite the names of all such non-defaulting Underwriters, or in such other proportions as the Representatives may specify, to purchase the Units that such defaulting Underwriter or Underwriters agreed but failed or refused to purchase on such date; provided that in no event shall the number of Units that any Underwriter has agreed to purchase pursuant to this Agreement be increased pursuant to this Section 11 by an amount in excess of one-ninth of such number of Units without the written consent of such Underwriter.  If, on the Closing Date, any Underwriter or Underwriters shall fail or refuse to purchase Firm Units and the aggregate number of Firm Units with respect to which such default occurs is more than one-tenth of the aggregate number of Firm Units to be purchased on such date, and arrangements satisfactory to the Representatives, the Selling Unitholder and the Partnership for the purchase of such Firm Units are not made within 36 hours after such default, this Agreement shall terminate without liability on the part of any non-defaulting Underwriter, the Selling Unitholder or the MarkWest Parties, except that the MarkWest Parties will continue to be liable for the payment of expenses to the extent set forth in Sections 7 and 10.  In any such case, the Representatives, the MarkWest Parties or the Selling Unitholder shall have the right to postpone the Closing Date, but in no event for longer than seven days, in order that the required changes, if any, in the Registration

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Statements, in the Pricing Disclosure Package, in the Prospectus or in any other documents or arrangements may be effected.  If, on an Option Closing Date, any Underwriter or Underwriters shall fail or refuse to purchase Additional Units and the aggregate number of Additional Units with respect to which such default occurs is more than one-tenth of the aggregate number of Additional Units to be purchased on such Option Closing Date, the non-defaulting Underwriters shall have the option to (i) terminate their obligation hereunder to purchase the Additional Units to be sold on such Option Closing Date or (ii) purchase not less than the number of Additional Units that such non-defaulting Underwriters would have been obligated to purchase in the absence of such default.  Any action taken under this paragraph shall not relieve any defaulting Underwriter from liability in respect of any default of such Underwriter under this Agreement.  As used in this Agreement, the term “Underwriter” includes, for all purposes of this Agreement unless the context requires otherwise, any party not listed in Schedule I hereto that, pursuant to this Section 11, purchases Units that a defaulting Underwriter agreed but failed to purchase.

If this Agreement shall be terminated by the Underwriters (other than as a result of the events described in Section 10 or this Section 11), or any of them, because of any failure or refusal on the part of the MarkWest Parties or the Selling Unitholder to comply with the terms or to fulfill any of the conditions of this Agreement, or if for any reason the MarkWest Parties or the Selling Unitholder shall be unable to perform their respective obligations under this Agreement, the MarkWest Parties will reimburse the Underwriters or such Underwriters as have so terminated this Agreement with respect to themselves, severally, for all out of pocket expenses (including the fees and disbursements of their counsel) reasonably incurred by such Underwriters in connection with this Agreement or the offering contemplated hereunder.

12.   Research Analyst Independence.  The MarkWest Parties and the Selling Unitholder acknowledge that the Underwriters’ research analysts and research departments are required to be independent from their respective investment banking divisions and are subject to certain regulations and internal policies, and that such Underwriters’ research analysts may hold views and make statements or investment recommendations and/or publish research reports with respect to the Partnership and/or the offering that differ from the views of their respective investment banking divisions.  The MarkWest Parties and the Selling Unitholder hereby waive and release, to the fullest extent permitted by law, any claims that the MarkWest Parties or the Selling Unitholder may have against the Underwriters with respect to any conflict of interest that may arise from the fact that the views expressed by their independent research analysts and research departments may be different from or inconsistent with the views or advice communicated to the MarkWest Parties or the Selling Unitholder by such Underwriters’ investment banking divisions.  The MarkWest Parties and the Selling Unitholder acknowledge that each of the Underwriters is a full service securities firm and as such from time to time, subject to applicable securities laws, may effect transactions for its own account or the account of its customers and hold long or short positions in debt or equity securities of the companies that may be the subject of the transactions contemplated by this Agreement.

13.   No Fiduciary Duty.  The MarkWest Parties and the Selling Unitholder acknowledge and agree that in connection with this offering, sale of the Units or any other services the Underwriters may be deemed to be providing hereunder, notwithstanding any preexisting relationship, advisory or otherwise, between the parties or any oral representations or assurances previously or subsequently made by the Underwriters:  (i) no fiduciary or agency

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relationship between the MarkWest Parties, the Selling Unitholder and any other person, on the one hand, and the Underwriters, on the other, exists; (ii) the Underwriters are not acting as advisors, expert or otherwise, to any of the MarkWest Parties or the Selling Unitholder, including, without limitation, with respect to the determination of the public offering price of the Units, and such relationship between the MarkWest Parties and the Selling Unitholder, on the one hand, and the Underwriters, on the other, is entirely and solely commercial, based on arms-length negotiations; (iii) any duties and obligations that the Underwriters may have to the MarkWest Parties or the Selling Unitholder shall be limited to those duties and obligations specifically stated herein; and (iv) the Underwriters and their respective affiliates may have interests that differ from those of the MarkWest Parties and the Selling Unitholder.  The MarkWest Parties and the Selling Unitholder hereby waive any claims that they may have against the Underwriters with respect to any breach of fiduciary duty in connection with this offering.

14.   Notices, Etc.   All communications hereunder shall be in writing and effective only upon receipt and shall be delivered, mailed or sent to the addresses set forth as follows:

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15.   Persons Entitled to Benefit of Agreement.   This Agreement shall inure to the benefit of and be binding upon the Underwriters, the Selling Unitholder, the MarkWest Parties and their respective successors.  This Agreement and the terms and provisions hereof are for the sole benefit of only those persons, except that (A) the representations, warranties, indemnities and agreements of the MarkWest Parties and the Selling Unitholder contained in this Agreement shall also be deemed to be for the benefit of the directors, officers and employees of the Underwriters and each person or persons, if any, who controls any Underwriter within the meaning of Section 15 of the Securities Act or Section 20 of the Exchange Act and (B) the representations, warranties, indemnities and agreements of the Underwriters contained in Section 6(c) and Section 9(c) of this Agreement shall be deemed to be for the benefit of the Selling Unitholder and the respective directors, managers and officers of the MarkWest Parties, the officers of the Partnership who have signed a Registration Statement and any person controlling the Partnership within the meaning of Section 15 of the Securities Act or Section 20 of the Exchange Act.  Nothing in this Agreement is intended or shall be construed to give any person, other than the persons referred to in this Section 15, any legal or equitable right, remedy or claim under or in respect of this Agreement or any provision contained herein.

16.   Survival.   The respective indemnities, representations, warranties and agreements of the MarkWest Parties, the Selling Unitholder and the Underwriters contained in this Agreement or made by or on behalf of them, respectively, pursuant to this Agreement, shall survive the delivery of and payment for the Units and shall remain in full force and effect, regardless of any investigation made by or on behalf of any of them or any person controlling any of them.

17.   Definition of the Terms “Business Day” and “Subsidiary”.   For purposes of this Agreement, (a) “business day” means each Monday, Tuesday, Wednesday, Thursday or Friday that is not a day on which banking institutions in New York are generally authorized or obligated by law or executive order to close and (b) “subsidiary” has the meaning set forth in Rule 405 of the Rules and Regulations.

18.   Governing Law.  This Agreement shall be governed by and construed in accordance with the laws of the State of New York.

19.   Counterparts.   This Agreement may be executed in one or more counterparts and, if executed in more than one counterpart, the executed counterparts shall each be deemed to be an original but all such counterparts shall together constitute one and the same instrument.

20.   Headings.   The headings herein are inserted for convenience of reference only and are not intended to be part of, or to affect the meaning or interpretation of, this Agreement.

[Signature Pages Follow]

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If the foregoing correctly sets forth the agreement between the MarkWest Parties and the Selling Unitholder, on the one hand, and the Underwriters, on the other hand, please indicate your acceptance in the space provided for that purpose below.

Signature Page to Underwriting Agreement

Accepted as of the date hereof:

MORGAN STANLEY & CO. INCORPORATED

UBS SECURITIES LLC

RBC CAPITAL MARKETS CORPORATION

Signature Page to Underwriting Agreement

SCHEDULE I

MarkWest Energy Partners, L.P.

SCHEDULE II

Free Writing Prospectuses

1.                                       Free Writing Prospectuses of the Partnership filed on August 13, 2009 pursuant to Rule 433 of the Securities Act (Film Numbers 091008571 and 091008572).

Additional Information:

SCHEDULE III

LOCK-UP LETTER AGREEMENT

August [  ], 2009

MORGAN STANLEY & CO. INCORPORATED

UBS SECURITIES LLC

AS REPRESENTATIVES OF THE

SEVERAL UNDERWRITERS

Ladies and Gentlemen:

The undersigned understands that Morgan Stanley & Co. Incorporated, UBS Securities LLC and RBC Capital Markets Corporation (the “Underwriters”) propose to enter into an Underwriting Agreement (the “Underwriting Agreement”) providing for the purchase by the Underwriters of common units representing limited partner interests (the “Common Units”) in MarkWest Energy Partners, L.P., a Delaware limited partnership (the “Partnership”), and that the Underwriters propose to reoffer such Common Units to the public (the “Offering”).

In consideration of the execution of the Underwriting Agreement by the Underwriters, and for other good and valuable consideration, the undersigned hereby irrevocably agrees that, without the prior written consent of Morgan Stanley & Co. Incorporated and UBS Securities LLC, as representatives of the Underwriters (the “Representatives”), the undersigned will not, directly or indirectly, (1) 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 Securities and Exchange 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