Exhibit 10.1

EXECUTION

FOURTH AMENDMENT TO AMENDED AND RESTATED CREDIT AGREEMENT

among

MARKWEST ENERGY PARTNERS, L.P.,

as the Borrower,

WELLS FARGO BANK, NATIONAL ASSOCIATION,

as Administrative Agent,

and

The Agents and Lenders from Time to Time Parties Thereto,

Dated as of December 20, 2012

WELLS FARGO SECURITIES, LLC

and

RBC CAPITAL MARKETS,

as Joint Lead Arrangers

and

WELLS FARGO SECURITIES, LLC,

and

RBC CAPITAL MARKETS

as Joint Bookrunners

FOURTH AMENDMENT TO MARKWEST ENERGY PARTNERS
AMENDED AND RESTATED CREDIT AGREEMENT

FOURTH AMENDMENT TO AMENDED AND RESTATED CREDIT AGREEMENT

THIS FOURTH AMENDMENT TO AMENDED AND RESTATED CREDIT AGREEMENT (herein called the “Amendment”) dated as of December 20, 2012 among MARKWEST ENERGY PARTNERS, L.P., a Delaware limited partnership (“Borrower”), WELLS FARGO BANK, NATIONAL ASSOCIATION, as Administrative Agent, Swingline Lender, and Issuing Bank, and the several banks and other financial institutions or entities from time to time parties to the Existing Credit Agreement defined below (“Lenders”).

W I T N E S S E T H:

WHEREAS, Borrower, Administrative Agent, Issuing Bank, Swingline Lender and Lenders entered into that certain Amended and Restated Credit Agreement dated as of July 1, 2010, as amended by a First Amendment to Amended and Restated Credit Agreement dated September 7, 2011, by a Second Amendment to Amended and Restated Credit Agreement dated December 29, 2011 and by a Third Amendment to Amended and Restated Credit Agreement dated June 29, 2012 (as amended, the “Existing Credit Agreement”), for the purpose and consideration therein expressed, whereby Lenders became obligated to make Revolving Loans to Borrower as therein provided; and

WHEREAS, Borrower, Administrative Agent, Issuing Bank, Swingline Lender and Lenders desire to amend the Existing Credit Agreement as set forth herein;

NOW, THEREFORE, in consideration of the premises and the mutual covenants and agreements contained herein and in the Existing Credit Agreement, in consideration of the loans which may hereafter be made by Lenders to Borrower, and for other good and valuable consideration, the receipt and sufficiency of which are hereby acknowledged, the parties hereto do hereby agree as follows:

ARTICLE I.

DEFINITIONS AND REFERENCES

Paragraph 1.1                Terms Defined in the Existing Credit Agreement.  Unless the context otherwise requires or unless otherwise expressly defined herein, the terms defined in the Existing Credit Agreement shall have the same meanings whenever used in this Amendment.

Paragraph 1.2                Other Defined Terms.  Unless the context otherwise requires, the following terms when used in this Amendment shall have the meanings assigned to them in this Paragraph 1.2.

“Amendment Documents” means this Amendment and all other documents or instruments delivered in connection herewith.

“Credit Agreement” means the Existing Credit Agreement as amended hereby.

“Fourth Amendment Effective Date” means December 20, 2012.

ARTICLE II.

AMENDMENTS TO EXISTING CREDIT AGREEMENT

Paragraph 2.1                Additional Defined Terms.  Section 1.01 of the Existing Credit Agreement is amended to add the following definitions:

“Fourth Amendment means that certain Fourth Amendment to Amended and Restated Credit Agreement dated as of the Fourth Amendment Effective Date, among the Borrower, the Guarantors, Wells Fargo Bank, National Association, as Administrative Agent, Issuing Bank and Swingline Lender and the Required Lenders.”

“Fourth Amendment Effective Date means December 20, 2012.”

Paragraph 2.2                Existing Defined Terms.

(a)                                 The following definitions in Section 1.01 of the Existing Credit Agreement are hereby amended in their entirety to read as follows:

“Agreement means this Agreement, which amends and restates in its entirety the Original Credit Agreement, as amended by the First Amendment, Second Amendment, Third Amendment and Fourth Amendment, as this Agreement may be further amended, modified, supplemented or restated from time to time in accordance with the terms hereof.”

“Loan Documents means this Agreement, the First Amendment, the Second Amendment, the Third Amendment, the Fourth Amendment, each Note, the Collateral Documents, the Issuing Documents, the Fee Letter, the First Amendment Fee Letter, the Second Amendment Fee Letter, the Third Amendment Fee Letter, any Loan Modification Agreement, and each and every other agreement executed in connection with this Agreement; provided, however, that in no event shall any Lender Hedging Agreement or any agreement in respect of Banking Services Obligations constitute a Loan Document hereunder.”

“MWLM&R Collateral Assets means (a) the natural gas processing facilities owned and operated by MWLM&R generally known and identified as Houston 1, Houston 2 and Houston 3 and (b) all plant, property and equipment related solely thereto and all contracts and revenues related solely thereto or generated solely thereby.”

(b)                                 The definition of Majorsville 4 in Section 1.01 of the Existing Credit Agreement is hereby deleted in its entirety.

Paragraph 2.3                Total Leverage Ratio.  Section 7.15(b) of the Existing Credit Agreement is hereby amended to read in its entirety as follows:

(b)                                 Total Leverage Ratio.  Permit the Total Leverage Ratio at any fiscal quarter-end through and including the fiscal quarter ending December 31, 2013, to be greater than 5.50 to 1.0 and thereafter at any fiscal quarter-end to be greater than 5.25 to 1.0; provided, (x) as a condition precedent to the Borrower’s designation of a Subsidiary as an Excluded Venture pursuant to Section 1.08 and (y) as a condition precedent to the Borrower making an Investment described in Section 7.02(i), the Total Leverage Ratio must not be greater than 4.75 to 1.0.

Paragraph 2.4                Adjustments for Material Projects.  The first sentence of Section 7.15(e)of the Existing Credit Agreement is hereby amended to read in its entirety as follows:

“For purposes of determining compliance with Sections 7.15(a), (b) and (c) in the event the Borrower, any of its consolidated Subsidiaries or any Excluded Venture, including MWLM&R, undertakes a Material Project, a Material Project Consolidated EBITDA

2

Adjustment may be made at Borrower’s option, subject to Administrative Agent’s review and approval of each component of such Material Project Consolidated EBITDA Adjustment.”

Paragraph 2.5                Adjustments for Material Projects.  Section 7.15(e)(iii) of the Existing Credit Agreement is hereby amended to read in its entirety as follows:

(iii)                               Notwithstanding the foregoing: (A) no such additions shall be allowed with respect to any Material Project unless: (x) the contracts relating to such Material Project are substantially fee-based, (y) not later than 15 days prior to the delivery of any Compliance Certificate required by the terms and provisions of Section 6.02(a) to the extent Material Project Consolidated EBITDA Adjustments will be made to Adjusted Consolidated EBITDA in determining compliance with this Section 7.15, the Borrower shall have delivered to the Administrative Agent a schedule comparable in scope and detail to the schedule attached to the prior quarter’s Compliance Certificate as “Worksheet 2 — Material Project Adjustments” and if requested by the Administrative Agent, written pro forma projections or models for such Material Projects as may be identified by Administrative Agent for which Material Project Consolidated EBITDA Adjustments are proposed, and (z) no later than 5 days prior to the date such Compliance Certificate is required to be delivered by the terms and provisions of Section 6.02(a), the Administrative Agent shall have approved (such approval not to be unreasonably withheld, conditioned or delayed) or be deemed to have approved such projections (deemed approval to occur if the Administrative Agent does not disapprove any such projection(s) in writing sent to the Borrower at least 5 days prior to the date such Compliance Certificate is required to be delivered), and (B) the aggregate amount of all Material Project Consolidated EBITDA Adjustments during any period shall be limited to 20% of the total actual Consolidated EBITDA of the Borrower and Excluded Ventures for such period (which total actual Consolidated EBITDA shall be determined without including any Material Project Consolidated EBITDA Adjustments).

Paragraph 2.6                            Amended Exhibit C.  Exhibit C “Compliance Certificate” to the Existing Credit Agreement is hereby deleted in its entirety and Exhibit C attached to this Amendment is substituted therefore.  Any reference in the Existing Credit Agreement to such Exhibit shall be deemed to refer to Exhibit C attached hereto.

ARTICLE III.

CONDITIONS OF EFFECTIVENESS

Paragraph 3.1                Fourth Amendment Effective Date(a).  This Amendment shall become effective as of the date first above written when and only when Administrative Agent shall have received this Amendment, executed by the Borrower, Lenders comprising Required Lenders, the Administrative Agent, Issuing Bank, and Swingline Lender and the Consent and Agreement attached to this Amendment executed by the Guarantors.

ARTICLE IV.

REPRESENTATIONS AND WARRANTIES

Paragraph 4.1                Representations and Warranties.  In order to induce the Required Lenders to enter into this Amendment, Borrower and each Guarantor represent and warrant to each Lender that:

3

(a)                                 The representations and warranties contained in Article V of the Existing Credit Agreement are true and correct in all material respects (except to the extent that any representation and warranty that is qualified by materiality shall be true and correct in all respects) at and as of the time of the effectiveness hereof, except to the extent such representations and warranties specifically refer to an earlier date, in which case they are true and correct in all material respects (except to the extent that any representation and warranty that is qualified by materiality shall be true and correct in all respects) as of such earlier date.

(b)                                 Borrower and Guarantors are duly authorized to execute and deliver this Amendment and the other Amendment Documents to which they are a party and Borrower is and will continue to be duly authorized to borrow monies and to perform its obligations under the Existing Credit Agreement. Borrower and Guarantors have duly taken all limited partnership, limited liability company or corporate action, as applicable, necessary to authorize the execution and delivery of this Amendment and the other Amendment Documents to which they are a party and, in the case of Borrower, to authorize the performance of the obligations of Borrower hereunder and thereunder.

(c)                                  When duly executed and delivered, each of this Amendment and the Existing Credit Agreement will be a legal, valid and binding obligation of Borrower, enforceable in accordance with its terms, except as limited by bankruptcy, insolvency or similar laws of general application relating to the enforcement of creditors’ rights and by equitable principles of general application.

(d)                                 No Default or Event of Default exists or will exist prior to and immediately after giving effect to this Amendment.

ARTICLE V.

MISCELLANEOUS

Paragraph 5.1                Ratification of Agreements.  The Existing Credit Agreement as hereby amended is and shall continue to be in full force and effect and is hereby ratified and confirmed in all respects.  The Loan Documents, as they may be amended or affected by the various Amendment Documents, are hereby ratified and confirmed in all respects. Any reference to the Existing Credit Agreement in any Loan Document shall be deemed to be a reference to the Existing Credit Agreement as hereby amended.  The execution, delivery and effectiveness of this Amendment and the other Amendment Documents shall not, except as expressly provided herein or therein, operate as a waiver of any right, power or remedy of Lenders under the Existing Credit Agreement, the Notes, or any other Loan Document nor constitute a waiver of any provision of the Existing Credit Agreement, the Notes or any other Loan Document.  Without limiting the generality of the foregoing, the Collateral Documents and all of the Collateral described therein do and shall continue to secure the payment of all Obligations of the Loan Parties under the Loan Documents, in each case, as amended by this Amendment.

Paragraph 5.2                            Survival of Agreements. All representations, warranties, covenants and agreements of Borrower and the Subsidiary Guarantors herein shall survive the execution and delivery of this Amendment and the performance hereof, including without limitation the making or granting of the Loans, and shall further survive until Payment in Full of the Obligations.

Paragraph 5.3                Loan Documents.  This Amendment, and each of the other Amendment Documents, is a Loan Document, and all provisions in the Credit Agreement pertaining to Loan Documents apply hereto and thereto.

4

Paragraph 5.4                Governing Law.  This Amendment shall be governed by and construed in accordance the laws of the State of New York and any applicable laws of the United States of America in all respects, including construction, validity and performance.

Paragraph 5.5                Miscellaneous.  This Amendment is a “Loan Document” referred to in the Credit Agreement.  The provisions relating to Loan Documents in Article X of the Credit Agreement are incorporated in this Amendment by reference.  Unless stated otherwise (a) the singular number includes the plural and vice versa and words of any gender include each other gender, in each case, as appropriate, (b) headings and captions may not be construed in interpreting provisions and (c) if any part of this Amendment is for any reason found to be unenforceable, all other portions of it nevertheless remain enforceable.

Paragraph 5.6                Release.  As additional consideration for the execution, delivery and performance of this Amendment by the parties hereto and to induce the Administrative Agent, Issuing Bank, Swingline Lender and the Lenders to enter into this Amendment, the Borrower warrants and represents to the Administrative Agent, Issuing Bank, Swingline Lender and the Lenders that to its knowledge no facts, events, statuses or conditions exist or have existed which, either now or with the passage of time or giving of notice, or both, constitute or will constitute a basis for any claim or cause of action against the Administrative Agent, Issuing Bank, Swingline Lender or any Lender or any defense to (i) the payment of the Obligations under the Notes and/or the Loan Documents, or (ii) the performance of any of its obligations with respect to the Notes and/or the Loan Documents.  In the event any such facts, events, statuses or conditions exist or have existed, Borrower unconditionally and irrevocably hereby RELEASES, RELINQUISHES and forever DISCHARGES Administrative Agent, Issuing Bank, Swingline Lender and the Lenders, as well as their predecessors, successors, assigns, agents, officers, directors, shareholders, employees and representatives, of and from any and all claims, demands, actions and causes of action of any and every kind or character, past or present, which Borrower may have against any of them or their predecessors, successors, assigns, agents, officers, directors, shareholders, employees and representatives arising out of or with respect to (a) any right or power to bring any claim for usury or to pursue any cause of action based on any claim of usury, and (b) any and all transactions relating to the Loan Documents occurring prior to the date hereof, including any loss, cost or damage, of any kind or character, arising out of or in any way connected with or in any way resulting from the acts, actions or omissions of any of them, and their predecessors, successors, assigns, agents, officers, directors, shareholders, employees and representatives, including any breach of fiduciary duty, breach of any duty of fair dealing, breach of confidence, breach of funding commitment, undue influence, duress, economic coercion, conflict of interest, negligence, bad faith, malpractice, intentional or negligent infliction of mental distress, tortious interference with contractual relations, tortious interference with corporate governance or prospective business advantage, breach of contract, deceptive trade practices, libel, slander or conspiracy, but in each case only to the extent permitted by applicable Law.

Paragraph 5.7                Counterparts; Fax.  This Amendment may be separately executed in counterparts and by the different parties hereto in separate counterparts, each of which when so executed shall be deemed to constitute one and the same Amendment.  This Amendment and the other Amendment Documents may be validly executed by facsimile or other electronic transmission. Delivery of an executed counterpart of a signature page of this Amendment by facsimile or other electronic transmission shall be effective as delivery of a manually executed counterpart of this Amendment.

THIS AMENDMENT AND THE OTHER LOAN DOCUMENTS REPRESENT THE FINAL AGREEMENT BETWEEN THE PARTIES AND MAY NOT BE CONTRADICTED BY EVIDENCE OF PRIOR, CONTEMPORANEOUS, OR SUBSEQUENT ORAL AGREEMENTS OF THE PARTIES.  THERE ARE NO UNWRITTEN ORAL AGREEMENTS OF THE PARTIES.

5

[The remainder of this page has been intentionally left blank.]

6

EXHIBIT C

FORM OF COMPLIANCE CERTIFICATE

Financial Statement Date:            

To:                             Wells Fargo Bank, National Association, as Administrative Agent

Ladies and Gentlemen:

Reference is made to that certain Amended and Restated Credit Agreement, dated as of July [1], 2010 (as amended, restated, extended, supplemented or otherwise modified in writing from time to time, the “Agreement;” the terms defined therein being used herein as therein defined), among MarkWest Energy Partners, L.P., a Delaware limited partnership (the “Borrower”), the Lenders from time to time party thereto, and Wells Fargo Bank, National Association, as Administrative Agent, Issuing Bank and Swingline Lender.

The undersigned Responsible Officer hereby certifies as of the date hereof that he/she is the                                                                  of the Borrower, and that, as such, he/she is authorized to execute and deliver this Certificate to the Administrative Agent on the behalf of the Borrower, and that:

[Use following paragraph 1 for Fiscal Year-end financial statements]

1.  Attached hereto as Schedule 1 are the year-end audited financial statements required by Section 6.01(a) of the Agreement for the Fiscal Year of the Borrower ended as of the above date, together with the report and opinion of an independent certified public accountant required by such section.

[Use following paragraph 1 for fiscal quarter-end financial statements]

1.                                      Attached hereto as Schedule 1 are the unaudited financial statements required by Section 6.01(b) of the Agreement for the fiscal quarter of the Borrower ended as of the above date. Such financial statements fairly present the financial condition, results of operations and cash flows of the Borrower and its Subsidiaries in all material respects on a consolidated basis in accordance with GAAP as at such date and for such period, subject only to normal year-end adjustments and the absence of footnotes.

2.                                      The undersigned has reviewed and is familiar with the terms of the Agreement and has made, or has caused to be made under his/her supervision, a detailed review of the transactions and condition (financial or otherwise) of the Borrower during the accounting period covered by the attached financial statements.

3.                                      A review of the activities of the Borrower during such fiscal period has been made under the supervision of the undersigned with a view to determining whether during such fiscal period the Borrower performed and observed all its Obligations under the Loan Documents, and

[select one:]

[to the knowledge of the undersigned during such fiscal period, the Borrower performed and observed each covenant and condition of the Loan Documents applicable to it.]

—or—

[the following covenants or conditions have not been performed or observed and the following is a list of each such Default and its nature and status:]

4.                                      The representations and warranties of the Borrower contained in Article V of the Agreement, and any representations and warranties of the Borrower that are contained in any document furnished by Borrower to the Administrative Agent or Lenders at any time under or in connection with the Loan Documents, are true and correct in all material respects on and as of the date hereof, except to the extent that such representations and warranties specifically refer to an earlier date, in which case they are true and correct as of such earlier date.

5.                                      The financial covenant analyses and information set forth on Schedule 2 attached hereto are true and accurate on and as of the date of this Certificate.

IN WITNESS WHEREOF, the undersigned has executed this Certificate as of                 ,201    .

FOURTH AMENDMENT TO MARKWEST ENERGY PARTNERS
AMENDED AND RESTATED CREDIT AGREEMENT

2

For the Quarter/Year ended

                             (“Statement Date”)

SCHEDULE 2

to the Compliance Certificate

($ in 000’s)

3

4

5

6

7

8

IN WITNESS WHEREOF, the parties hereto have caused this Amendment to be duly executed and delivered by their proper and duly authorized officers as of the day and year first above written.

FOURTH AMENDMENT TO MARKWEST ENERGY PARTNERS

AMENDED AND RESTATED CREDIT AGREEMENT

SIGNATURE  PAGE

1

FOURTH AMENDMENT TO MARKWEST ENERGY PARTNERS

AMENDED AND RESTATED CREDIT AGREEMENT

SIGNATURE  PAGE

2

FOURTH AMENDMENT TO MARKWEST ENERGY PARTNERS

AMENDED AND RESTATED CREDIT AGREEMENT

SIGNATURE  PAGE

3

FOURTH AMENDMENT TO MARKWEST ENERGY PARTNERS

AMENDED AND RESTATED CREDIT AGREEMENT

SIGNATURE  PAGE

4

FOURTH AMENDMENT TO MARKWEST ENERGY PARTNERS

AMENDED AND RESTATED CREDIT AGREEMENT

SIGNATURE  PAGE

5

Fourth Amendment

CONSENT AND AGREEMENT

Each of the undersigned (in their individual capacity, each a “Guarantor”), as of the Fourth Amendment Effective Date hereby (i) consents to the provisions of this Amendment and the transactions contemplated herein, (ii) ratifies and confirms the Amended and Restated Guaranty dated as of July 1, 2010 made by it for the benefit of Administrative Agent and Lenders executed pursuant to the Existing Credit Agreement and the other Loan Documents, (iii) agrees that all of its respective obligations and covenants thereunder shall remain unimpaired by the execution and delivery of this Amendment and the other documents and instruments executed in connection herewith, and (iv) agrees that the Amended and Restated Guaranty and such other Loan Documents shall remain in full force and effect.

FOURTH AMENDMENT TO MARKWEST ENERGY PARTNERS

AMENDED AND RESTATED CREDIT AGREEMENT

CONSENT AND AGREEMENT

1

FOURTH AMENDMENT TO MARKWEST ENERGY PARTNERS

AMENDED AND RESTATED CREDIT AGREEMENT

CONSENT AND AGREEMENT

2

FOURTH AMENDMENT TO MARKWEST ENERGY PARTNERS

AMENDED AND RESTATED CREDIT AGREEMENT

CONSENT AND AGREEMENT

3

FOURTH AMENDMENT TO MARKWEST ENERGY PARTNERS

AMENDED AND RESTATED CREDIT AGREEMENT

CONSENT AND AGREEMENT

4

FOURTH AMENDMENT TO MARKWEST ENERGY PARTNERS

AMENDED AND RESTATED CREDIT AGREEMENT

CONSENT AND AGREEMENT

5