ITEM 1.01. Entry into a Material Definitive Agreement

On October 30, 2009, MarkWest Energy Partners, L.P. (the "Partnership"), through its wholly-owned subsidiary MarkWest Liberty Gas Gathering, L.L.C. ("MWE Liberty"), and NGP Midstream & Resources, L.P. and its affiliated funds ("M&R"), through its affiliate M&R MWE Liberty, LLC, closed on a modification to the terms of their joint venture, which is operated through MarkWest Liberty Midstream & Resources, L.L.C. ("MarkWest Liberty Midstream"). The proposed modification of the terms of their joint venture was originally announced in a Current Report on Form 8-K filed on August 12, 2009. Pursuant to the Second Amended and Restated Limited Liability Company Agreement executed on October 30, 2009 and effective November 1, 2009 (the "Amended LLC Agreement"), M&R will increase its participation and ownership in MarkWest Liberty Midstream and M&R will increase its initial investment by an additional $150.0 million. Under the Amended LLC Agreement, the Partnership and M&R will maintain a 60%/40% respective ownership interest in MarkWest Liberty Midstream until January 1, 2011, at which time M&R's ownership interest will increase from 40% to 49%. The Partnership and M&R will jointly fund the capital requirements of MarkWest Liberty Midstream at agreed upon levels until the Partnership's contributed capital is proportionate to its 51% ownership interest (the "Equalization Date"), which is expected to occur on or before December 31, 2012. Following the Equalization Date, M&R will have pre-emptive rights to maintain its ownership interest in MarkWest Liberty Midstream in a range of between 45% and 49%.

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