UNITED STATES
SECURITIES AND EXCHANGE COMMISSION

Washington, D.C. 20549

FORM 8-K

CURRENT REPORT

PURSUANT TO SECTION 13 OR 15(d) OF

THE SECURITIES EXCHANGE ACT OF 1934

Date of Report (Date of earliest event reported): June 29, 2012

MARKWEST ENERGY PARTNERS, L.P.

(Exact name of registrant as specified in its charter)

1515 Arapahoe Street, Tower 1, Suite 1600, Denver CO 80202

(Address of principal executive offices)

Registrant’s telephone number, including area code: 303-925-9200

Not Applicable.

(Former name or former address, if changed since last report.)

Check the appropriate box below if the Form 8-K filing is intended to simultaneously satisfy the filing obligation of the registrant under any of the following provisions (see General Instruction A.2. below):

o Written Communications pursuant to Rule 425 under the Securities Act (17 CFR 230.425)

o Soliciting material pursuant to Rule 14a-12 under the Exchange Act (17 CFR 240.14a-12)

o    Pre-Commencement communications pursuant to Rule 14d-2(b) under the Exchange Act (17 CFR 240.14d-2(b))

o Pre-Commencement communications pursuant to Rule 13e-4(c) under the Exchange Act (17 CFR 240.13e-4(c))

ITEM 1.01.          Entry into a Material Definitive Agreement.

Credit Agreement Amendment

On June 29, 2012, MarkWest Energy Partners, L.P. (the “Partnership”) entered into the Third Amendment to the Amended and Restated Credit Agreement with Wells Fargo Bank, National Association as Administrative Agent and Collateral Agent (the “Agent”), and the other agents and lenders that are party thereto (the “Credit Agreement Amendment”), which amends the Amended and Restated Credit Agreement dated as of July 1, 2010, among the Agent, the other agents and lenders that are party thereto from time to time and the Partnership, as supplemented by the Joinder Agreement dated July 29, 2010 and the Joinder Agreement dated June 15, 2011, and as amended by the First Amendment to Amended and Restated Credit Agreement dated September 7, 2011 and the Second Amendment to Amended and Restated Credit Agreement dated December 29, 2011 (the “Existing Credit Agreement”).  Pursuant to the Credit Agreement Amendment, the commitments under the revolving loan facility were increased from $900 million to $1.2 billion (the “Facility”) and the maturity date for the Facility was extended by one year to September 7, 2017.  In addition, the existing accordion option, providing for potential further increases of the Facility commitments of up to $250 million in the event that any existing or new lender is willing to provide additional commitments was retained, with the effect that the exercise of such option would allow for aggregate commitments under the Facility of up to $1.45 billion.

The description of the Credit Agreement Amendment does not purport to be complete and is qualified in its entirety by reference to the complete text of such agreement, a copy of which is filed as Exhibit 10.1 to this current report on Form 8-K and is incorporated herein by reference.

ITEM 2.03.          Creation of a Direct Financial Obligation or an Obligation under an Off-Balance Sheet Arrangement of a Registrant.

The information included in Item 1.01 above under the heading “Credit Agreement Amendment” is incorporated by reference into this Item 2.03.

ITEM 7.01.          Regulation FD Disclosure.

On June 29, 2012, the Partnership issued a press release announcing the Credit Agreement Amendment. A copy of the press release is furnished as Exhibit 99.1 hereto.

The information contained in this Item 7.01 shall not be deemed “filed” for purposes of Section 18 of the Securities Exchange Act of 1934, as amended (the “Exchange Act”), or incorporated by reference in any filing under the Securities Act of 1933, as amended, or the Exchange Act, except as shall be expressly set forth by specific reference in such filing.

Information contained in this Current Report on Form 8-K may include certain statements concerning expectations for the future that are forward-looking statements as defined by federal law. Actual results could vary significantly from those expressed or implied in such statements and are subject to a variety of known and unknown risks, uncertainties, and other factors that are difficult to predict and many of which are beyond management’s control. Among the factors that could cause results to differ materially are those risks discussed in the periodic reports we file with the Securities and Exchange Commission, including our Annual Report on Form 10-K for the year ended December 31, 2011, and our Quarterly Report on Form 10-Q for the quarter ended March 31, 2012. You are urged to carefully review and consider the cautionary statements and other disclosures made in those filings, specifically those under the heading “Risk Factors.”  We do not undertake any duty to update any forward-looking statement except as required by law.

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ITEM 9.01.          Financial Statements and Exhibits.

(d)           Exhibits.

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SIGNATURE

Pursuant to the requirements of the Securities and Exchange Act of 1934, the Registrant has duly caused this report to be signed on its behalf by the undersigned, thereunto duly authorized.

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