News Release

DENVER--(BUSINESS WIRE)--May. 14, 2012-- MarkWest Energy Partners, L.P. (NYSE: MWE) announced today the closing of its previously announced public offering of 8,000,000 common units at $55.28 per common unit. MarkWest has granted the underwriters a 30-day option to purchase up to 1,200,000 additional common units. MarkWest intends to use the net proceeds from the offering, after deducting underwriting discounts, commissions and offering expenses, of approximately $427.4 million, and any additional net proceeds from the exercise of the underwriters’ option, to partially fund its acquisition of Keystone Midstream Services, LLC, which owns certain gas gathering and processing facilities located in Butler County, Pennsylvania (the “Keystone Acquisition”). If MarkWest does not consummate the Keystone Acquisition, it intends to use the net proceeds from the offering, as well as any exercise of the underwriters’ option for general partnership purposes, including, but not limited to, funding capital expenditures and general working capital.

Morgan Stanley, BofA Merrill Lynch, Barclays, Citigroup, UBS Investment Bank, and Wells Fargo Securities acted as joint book-running managers for the offering. A copy of the prospectus supplement and accompanying base prospectus associated with this offering may be obtained from the underwriters as follows:

An electronic copy of the preliminary prospectus supplement and the accompanying base prospectus is available on the Securities and Exchange Commission’s (SEC) web site at http://www.sec.gov. The common units are being offered pursuant to an effective shelf registration statement that MarkWest previously filed with the SEC.

This press release shall not constitute an offer to sell or the solicitation of an offer to buy nor shall there be any sale of these securities in any state in which such offer, solicitation, or sale would be unlawful prior to registration or qualification under the securities laws of such state.

MarkWest Energy Partners, L.P. is a master limited partnership engaged in the gathering, transportation, and processing of natural gas; the transportation, fractionation, marketing, and storage of natural gas liquids; and the gathering and transportation of crude oil. MarkWest has extensive natural gas gathering, processing, and transmission operations in the southwest, Gulf Coast, and northeast regions of the United States, including the Marcellus Shale, and is the largest natural gas processor and fractionator in the Appalachian region.

This press release includes “forward-looking statements.” All statements other than statements of historical facts included or incorporated herein may constitute forward-looking statements. Actual results could vary significantly from those expressed or implied in such statements and are subject to a number of risks and uncertainties, including the possibility that the Keystone Acquisition will not be consummated. Although MarkWest believes that the expectations reflected in the forward-looking statements are reasonable, MarkWest can give no assurance that such expectations will prove to be correct. The forward-looking statements involve risks and uncertainties that affect operations, financial performance, and other factors as discussed in filings with the Securities and Exchange Commission. Among the factors that could cause results to differ materially are those risks discussed in the periodic reports filed with the SEC, including MarkWest’s Annual Report on Form 10-K for the year ended December 31, 2011, and its 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.” MarkWest does not undertake any duty to update any forward-looking statement except as required by law.

Source: MarkWest Energy Partners, L.P.

MarkWest Energy Partners, L.P.
Frank Semple, 866-858-0482
Chairman, President & CEO
or
Nancy Buese, 866-858-0482
Senior VP & CFO
or
Josh Hallenbeck, 866-858-0482
VP of Finance & Treasurer
investorrelations@markwest.com