Exhibit 99.1

MarkWest Energy Partners Reports Second Quarter Financial Results
and Increases Common Unit Distribution by 14.3 Percent

DENVER—August 2, 2012—MarkWest Energy Partners, L.P. (NYSE: MWE) (the Partnership) today reported quarterly cash available for distribution to common unitholders, or distributable cash flow (DCF), of $91.2 million for the three months ended June 30, 2012, and $200.4 million for the six months ended June 30, 2012.  Distributable cash flow for the three months ended June 30, 2012, represents distribution coverage of 103 percent.  The second quarter distribution of $88.6 million, or $0.80 per common unit, will be paid to unitholders on August 14, 2012. The second quarter 2012 distribution represents an increase of $0.01 per common unit, or 1.3 percent, over the first quarter 2012 distribution and an increase of $0.10 per common unit, or 14.3 percent, over the second quarter 2011 distribution.  As a Master Limited Partnership, cash distributions to common unitholders are largely determined based on DCF.  A reconciliation of DCF to net income, the most directly comparable GAAP financial measure, is provided within the financial tables of this press release.

The Partnership reported Adjusted EBITDA for the three and six months ended June 30, 2012, of $130.5 million and $263.5 million, respectively, as compared to $120.0 million and $216.2 million for the three and six months ended June 30, 2011.  The Partnership believes the presentation of Adjusted EBITDA provides useful information because it is commonly used by investors in Master Limited Partnerships to assess financial performance and operating results of ongoing business operations.  A reconciliation of Adjusted EBITDA to net income, the most directly comparable GAAP financial measure, is provided within the financial tables of this press release.

The Partnership reported income before provision for income tax for the three and six months ended June 30, 2012, of $231.6 million and $252.4 million, respectively.  Income before provision for income tax includes non-cash gain associated with the change in mark-to-market of derivative instruments of $193.7 million and $145.5 million for the three and six months ended June 30, 2012, respectively.  Excluding these items, income before provision for income tax for the three and six months ended June 30, 2012, would have been $37.9 million and $106.9 million, respectively.

“Our mid-year results reflect the continued solid performance of our core assets despite the significant decrease in processing margins and natural gas liquids prices during the second quarter,” said Frank Semple, Chairman, President and Chief Executive Officer. “Our year-over-year processed volumes have increased by 18% as a result of our operational performance and ongoing growth projects located in some of the best resource plays in the United States.  During the second quarter, we also improved our liquidity position and capital flexibility with a $300 million increase to our credit facility and an equity offering with net proceeds over $427 million.  The combination of MarkWest’s high-quality assets, significant fee-based growth opportunities, and balance sheet strength supports our objective of providing long term sustainable top-quartile returns for our unitholders.”

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BUSINESS HIGHLIGHTS

Business Development

·                  Keystone Midstream Services Acquisition:  In May 2012, the Partnership completed the acquisition of 100% of the ownership interests of Keystone Midstream Services, LLC (Keystone), for consideration of $509.6 million, adjusted for working capital.  Keystone was owned by Stonehenge Energy Resources, LP, and affiliates of Rex Energy Corporation (Rex Energy), and Sumitomo Corporation (Sumitomo).  Keystone’s existing assets are located in Butler County, Pennsylvania and include two cryogenic gas processing plants totaling 90 million cubic feet per day (MMcf/d) of capacity, a gas gathering system and associated field compression.  Rex Energy and Sumitomo have dedicated approximately 900 square miles to the Partnership.  The parties have jointly leased 68,400 highly prospective acres in Butler County. The Partnership will gather and process the rich gas and fractionate the natural gas liquids (NGLs) under long-term fee-based agreements.

Pursuant to a letter agreement signed at the time of the Keystone acquisition, MarkWest Utica EMG, LLC (MarkWest Utica) is evaluating  gathering, processing, and NGL fractionation opportunities for portions of Rex Energy’s Ohio Utica acreage.

·                  Liberty:  In May 2012, the Partnership announced additional major expansion projects to serve producer customers in the hydrocarbon-rich area of the Marcellus Shale in northern West Virginia and southwest Pennsylvania area, including another 400 MMcf/d expansion of its Majorsville processing complex which includes two, 200 MMcf/d processing plants that are expected to be completed in late 2013 and mid 2014 and are supported by long-term agreements with Chesapeake Energy.  Considering the expansions announced in January and May 2012, the Partnership will have 1.1 billion cubic feet per day of cryogenic processing capacity at its Majorsville processing complex.

In May 2012, the Partnership announced a long-term fee-based agreement with Antero Resources Appalachian Corporation (Antero) to install gathering facilities in support of Antero’s rapidly growing rich natural gas production in Doddridge and Harrison Counties in northern West Virginia.  The new gathering system will have the capacity to initially deliver more than 300 MMcf/d of Antero’s rich gas to the Partnership’s Sherwood gas processing complex. The first phase of the gathering system will be completed in the third quarter of 2012 in conjunction with the completion of the 200 MMcf/d Sherwood I processing facility.

In May 2012, the Partnership also announced that it is extending its existing NGL gathering pipeline from its Houston, Pennsylvania fractionation complex into Beaver, Butler and Lawrence Counties to gather NGLs from the Keystone processing facilities and other planned processing projects in Northwest Pennsylvania.  The NGL pipeline expansion will allow Rex Energy and other producers to access all of the anticipated ethane pipeline projects.

In July 2012, the Partnership announced a new long-term, fee-based agreement with XTO Energy (XTO) to transport, fractionate and market NGLs from their 125 MMcf/d processing plant located in Butler County, Pennsylvania, which is expected to be operational in late 2012.  NGLs will initially be transported by truck from XTO’s plant to the Houston fractionation complex.  By the end of 2013, an extension of the Partnership’s NGL gathering pipeline is expected to be complete and will connect the Keystone processing facilities to the XTO facility.

In late June, the Partnership began delivering propane to Sunoco Inc.’s (Sunoco) Marcus Hook facility located outside Philadelphia, Pennsylvania.  Propane is currently being

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transported by truck from the Houston fractionation complex to Marcus Hook, with rail deliveries expected to be added in the next several months. In addition, the Partnership is purchasing propane produced at Sunoco’s local-area facilities and the combined stream is being loaded onto ships for marketing by the Partnership to international customers.  The delivery of propane from the northeast U.S. to global markets is critical to ensure northeast propane markets remain in balance and that northeast propane continues to achieve premium pricing.  This milestone is part of the Partnership’s ongoing commitment to provide multiple marketing options that will maximize the value of its producer customers’ NGLs.

Utica:  In June 2012, MarkWest Utica announced the completion of definitive agreements with Gulfport Energy Corporation to provide gathering, processing, fractionation, and marketing services primarily in Harrison, Guernsey, Belmont and Noble counties of Ohio in the liquids-rich window of the Utica Shale.  MarkWest Utica expects the first phase to be operational beginning in the third quarter of 2012.  It is anticipated MarkWest Utica will have approximately 60 miles of gas gathering pipelines and associated compression to move Gulfport volumes by the end of 2012 and up to 140 miles of gathering pipelines by the first quarter of 2014.  MarkWest Utica anticipates refrigeration processing capacity of 105 MMcf/d by the end of 2012 and 125 MMcf/d of cryogenic processing capacity in early 2013, increasing to 325 MMcf/d of total cryogenic processing capacity by the end of 2013.  The NGLs from these processing plants, as well as from our Marcellus operations, will ultimately be fractionated at the previously announced 100,000 Bbl/d Harrison County fractionation complex.

Capital Markets

·                  On May 14, 2012, the Partnership completed a common unit equity offering of 8.0 million common units. The net proceeds of approximately $427.2 million were used to partially fund the acquisition of Keystone Midstream.

·                  On June 29, 2012, the Partnership executed an amendment to its senior secured revolving credit facility, which increased total borrowing capacity by $300.0 million to $1.2 billion and extended the maturity by one year to September 2017.

FINANCIAL RESULTS

Balance Sheet

·                  At June 30, 2012, the Partnership had $121.7 million of cash and cash equivalents in wholly owned subsidiaries and $959.8 million available for borrowing under its $1.2 billion revolving credit facility after consideration of aggregate borrowings of $217.9 million and $22.3 million of outstanding letters of credit.

Operating Results

·                  Operating income before items not allocated to segments for the three months ended June 30, 2012, was $146.3 million, a decrease of $1.5 million when compared to segment operating income of $147.8 million in the same period in 2011.  This decrease is primarily attributable to lower commodity prices compared to the prior year quarter.  Processed volumes remain strong, growing almost 20% when compared to the second quarter of 2011, primarily due to the Partnership’s Southwest and Liberty segments.

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A reconciliation of operating income before items not allocated to segments to income (loss) before provision for income tax, the most directly comparable GAAP financial measure, is provided within the financial tables of this press release.

·                  Operating income before items not allocated to segments does not include loss on commodity derivative instruments.  Realized losses on commodity derivative instruments were $5.0 million in the second quarter of 2012 compared to realized losses of $17.7 million in the second quarter of 2011.

Capital Expenditures

·                  For the three and six months ended June 30, 2012, the Partnership’s portion of capital expenditures was $327.9 million and $582.2 million, respectively.  These expenditures do not include the Keystone purchase price of $509.6 million.

2012 DCF AND GROWTH CAPITAL EXPENDITURE FORECAST

For 2012, the Partnership forecasts DCF in a range of $400 million to $440 million based on its current forecast of operational volumes and prices for crude oil, natural gas and natural gas liquids; derivative instruments currently outstanding; and the Keystone acquisition, as mentioned above.  The midpoint of this range results in approximately 119 percent coverage of the Partnership’s full-year distribution based on current quarterly distributions and common units outstanding.  A commodity price sensitivity analysis for forecasted 2012 DCF is provided within the tables of this press release.

The Partnership’s portion of growth capital expenditures for 2012 remains unchanged and is forecasted in a range of $1.1 billion to $1.5 billion.  This range excludes the Keystone purchase price of $509.6 million.  Maintenance capital for 2012 is forecasted at approximately $20 million.

CONFERENCE CALL

The Partnership will host a conference call and webcast on Friday, August 3, 2012, at 12:00 p.m. Eastern Time to review its second quarter 2012 financial results. Interested parties can participate in the call by dialing (800) 475-0218 (passcode “MarkWest”) approximately ten minutes prior to the scheduled start time.  To access the webcast, please visit the Investor Relations section of the Partnership’s website at www.markwest.com. A replay of the conference call will be available on the MarkWest website or by dialing (866) 463-4105 (no passcode required).

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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. Although MarkWest believes that the expectations reflected in the forward-looking statements are reasonable, it can give no assurance that such expectations will prove to be correct. The forward-looking statements involve risks and uncertainties that affect MarkWest’s operations, financial performance, and other factors as discussed in its 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

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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.

MarkWest Energy Partners, L.P.

Financial Statistics

(unaudited, in thousands, except per unit data)

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MarkWest Energy Partners, L.P.

Operating Statistics

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MarkWest Energy Partners, L.P.

Reconciliation of GAAP Financial Measure to Non-GAAP Financial Measure

Operating Income before Items not Allocated to Segments

(unaudited, in thousands)

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MarkWest Energy Partners, L.P.

Reconciliation of GAAP Financial Measure to Non-GAAP Financial Measure

Operating Income before Items not Allocated to Segments

(unaudited, in thousands)

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MarkWest Energy Partners, L.P.

Reconciliation of GAAP Financial Measure to Non-GAAP Financial Measure

Distributable Cash Flow

(unaudited, in thousands)