X0306 4/A 2012-10-01 2012-10-01 0 0001166036 MARKWEST ENERGY PARTNERS L P MWE 0001235082 MOLLENKOPF JOHN C C/O MARKWEST ENERGY PARTNERS, L.P. 1515 ARAPAHOE ST. TWR 1, STE 1600 DENVER CO 80202 0 1 0 0 SVP, Chief Operating Officer Common Units 311870 D On October 1, 2012, the reporting person mistakenly filed a Form 4 reporting a sale of 10,000 common units that did not in fact occur, due to a broker error. As of October 1, 2012, the reporting person owned 311,870 units. Amount of securities beneficially owned following the reported transaction(s) includes Common Units owned and Restricted Units subject to varying vesting terms. /s/ Sean M. McKendry as POA for John Mollenkopf 2012-10-16