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<p align="center" style="margin:0in 0in .0001pt;text-align:center;"><b><font size="5" face="Times New Roman" style="font-size:18.0pt;font-weight:bold;">UNITED STATES</font></b></p>
<p align="center" style="margin:0in 0in .0001pt;text-align:center;"><b><font size="5" face="Times New Roman" style="font-size:18.0pt;font-weight:bold;">SECURITIES AND EXCHANGE COMMISSION</font></b></p>
<p align="center" style="margin:0in 0in .0001pt;text-align:center;"><b><font size="2" face="Times New Roman" style="font-size:10.0pt;font-weight:bold;">Washington, D.C. 20549</font></b></p>
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<p align="center" style="margin:0in 0in .0001pt;text-align:center;"><b><font size="5" face="Times New Roman" style="font-size:18.0pt;font-weight:bold;">FORM&nbsp;8-K</font></b></p>
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<p align="center" style="margin:0in 0in .0001pt;text-align:center;"><b><font size="3" face="Times New Roman" style="font-size:12.0pt;font-weight:bold;">CURRENT REPORT</font></b></p>
<p align="center" style="margin:0in 0in .0001pt;text-align:center;"><b><font size="3" face="Times New Roman" style="font-size:12.0pt;font-weight:bold;">PURSUANT TO SECTION&nbsp;13 OR 15(d)&nbsp;OF</font></b></p>
<p align="center" style="margin:0in 0in .0001pt;text-align:center;"><b><font size="3" face="Times New Roman" style="font-size:12.0pt;font-weight:bold;">THE SECURITIES EXCHANGE ACT OF 1934</font></b></p>
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<p align="center" style="margin:0in 0in .0001pt;text-align:center;"><font size="2" face="Times New Roman" style="font-size:10.0pt;">Date of Report (Date of earliest event reported):<b> August&nbsp;7, 2013</b></font></p>
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<p align="center" style="margin:0in 0in .0001pt;text-align:center;"><b><font size="5" face="Times New Roman" style="font-size:18.0pt;font-weight:bold;">MARKWEST ENERGY PARTNERS, L.P.</font></b></p>
<p align="center" style="margin:0in 0in .0001pt;text-align:center;"><font size="2" face="Times New Roman" style="font-size:10.0pt;">(Exact name of registrant as specified in its charter)</font></p>
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<p align="center" style="margin:0in 0in .0001pt;text-align:center;"><b><font size="2" face="Times New Roman" style="font-size:10.0pt;font-weight:bold;">Delaware</font></b></p>    </td>
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<p align="center" style="margin:0in 0in .0001pt;text-align:center;"><b><font size="2" face="Times New Roman" style="font-size:10.0pt;font-weight:bold;">001-31239</font></b></p>    </td>
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<p align="center" style="margin:0in 0in .0001pt;text-align:center;"><b><font size="2" face="Times New Roman" style="font-size:10.0pt;font-weight:bold;">27-0005456</font></b></p>    </td>   </tr>
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<p align="center" style="margin:0in 0in .0001pt;text-align:center;"><font size="2" face="Times New Roman" style="font-size:10.0pt;">(State or other jurisdiction of<br>   incorporation or organization)</font></p>    </td>
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<p align="center" style="margin:0in 0in .0001pt;text-align:center;"><font size="2" face="Times New Roman" style="font-size:10.0pt;">(I.R.S. Employer<br>   Identification Number)</font></p>    </td>   </tr>  </table>
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<p style="margin:0in 0in .0001pt;"><font size="2" face="Times New Roman" style="font-size:10.0pt;">&nbsp;</font></p>
<p align="center" style="margin:0in 0in .0001pt;text-align:center;"><b><font size="2" face="Times New Roman" style="font-size:10.0pt;font-weight:bold;">1515 Arapahoe Street, Tower 1, Suite&nbsp;1600, Denver CO 80202</font></b></p>
<p align="center" style="margin:0in 0in .0001pt;text-align:center;"><font size="2" face="Times New Roman" style="font-size:10.0pt;">(Address of principal executive offices)</font></p>
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<p align="center" style="margin:0in 0in .0001pt;text-align:center;"><font size="2" face="Times New Roman" style="font-size:10.0pt;">Registrant&#146;s telephone number, including area code: <b>303-925-9200</b></font></p>
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<p align="center" style="margin:0in 0in .0001pt;text-align:center;"><b><font size="2" face="Times New Roman" style="font-size:10.0pt;font-weight:bold;">Not Applicable.</font></b></p>
<p align="center" style="margin:0in 0in .0001pt;text-align:center;"><font size="2" face="Times New Roman" style="font-size:10.0pt;">(Former name or former address, if changed since last report.)</font></p>
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<p style="margin:0in 0in .0001pt;"><font size="2" face="Times New Roman" style="font-size:10.0pt;">Check the appropriate box below if the Form&nbsp;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):</font></p>
<p style="margin:0in 0in .0001pt;"><font size="2" face="Times New Roman" style="font-size:10.0pt;">&nbsp;</font></p>
<p style="margin:0in 0in .0001pt .25in;"><font size="2" face="Wingdings" style="font-size:10.0pt;">o</font><font size="2" style="font-size:10.0pt;">&#160;&#160;&#160; Written Communications pursuant to Rule&nbsp;425 under the Securities Act (17 CFR 230.425)</font></p>
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<p style="margin:0in 0in .0001pt .25in;"><font size="2" face="Wingdings" style="font-size:10.0pt;">o</font><font size="2" style="font-size:10.0pt;">&#160;&#160;&#160; Soliciting material pursuant to Rule&nbsp;14a-12 under the Exchange Act (17 CFR 240.14a-12)</font></p>
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<p style="margin:0in 0in .0001pt .5in;text-indent:-.25in;"><font size="2" face="Wingdings" style="font-size:10.0pt;">o</font><font size="1" style="font-size:3.0pt;">&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160; </font><font size="2" style="font-size:10.0pt;">Pre-Commencement communications pursuant to Rule&nbsp;14d-2(b)&nbsp;under the Exchange Act (17 CFR 240.14d-2(b))</font></p>
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<p style="margin:0in 0in .0001pt .5in;text-indent:-.25in;"><font size="2" face="Wingdings" style="font-size:10.0pt;">o</font><font size="1" style="font-size:3.0pt;">&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160; </font><font size="2" style="font-size:10.0pt;">Pre-Commencement communications pursuant to Rule&nbsp;13e-4(c)&nbsp;under the Exchange Act (17 CFR 240.13e-4(c))</font></p>
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<p style="margin:0in 0in .0001pt;"><b><font size="2" face="Times New Roman" style="font-size:10.0pt;font-weight:bold;">ITEM 1.01.&nbsp;&nbsp;&nbsp;&nbsp;Entry into a Material Agreement.</font></b></p>
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<p style="margin:0in 0in .0001pt;text-indent:29.7pt;"><font size="2" face="Times New Roman" style="font-size:10.0pt;">On August&nbsp;7, 2013, MarkWest Energy Partners, L.P. (the &#147;Partnership&#148;), MarkWest Energy Operating Company, L.L.C. (the &#147;Operating Company&#148;) and M&amp;R MWE Liberty, LLC (the &#147;Selling Unitholder&#148;) entered into an Equity Distribution Agreement (the &#147;Equity Distribution Agreement&#148;) with Citigroup Global Markets Inc. (the &#147;Sales Agent&#148;). Pursuant to the Equity Distribution Agreement, the Partnership may sell from time to time through the Sales Agent common units representing limited partner interests (the &#147;Common Units&#148;) having an aggregate offering price of up to $400,000,000 (the &#147;Primary Offering&#148;).&#160; In addition, the Selling Unitholder may sell from time to time through the Sales Agent up to 1,452,415 Common Units (the &#147;Secondary Offering&#148; and, together with the Primary Offering, the &#147;Common Unit Offering&#148;).&#160; Such sales under the Secondary Offering will be made concurrently with the sales of the Primary Offering.&#160; The Selling Unitholder has granted the Partnership a stock power (the &#147;Stock Power&#148;) relating to all of the Selling Unitholder&#146;s Common Units.&#160; Pursuant to this Stock Power, up to 10% of the Common Units disposed of in any Common Unit Offering will be sold on behalf of the Selling Unitholder, and the remaining Common Units disposed of in any such sale will be sold on behalf of the Partnership.&#160; The Common Units to be sold in the Primary Offering are registered under the Securities Act of 1933, as amended (the &#147;Securities Act&#148;), pursuant to a registration statement on Form&nbsp;S-3 (File No.&nbsp;333-184482).&#160; The Common Units to be sold in the Secondary Offering are registered under the Securities Act pursuant to a registration statement on Form&nbsp;S-3 (File No.&nbsp;333-186011).&#160; Sales of the Common Units, if any, will be made by means of ordinary brokers&#146; transactions on the New York Stock Exchange at market prices, in block transactions or as otherwise agreed by the Partnership and the Sales Agent.&#160; Legal opinions related to the Common Units are filed as Exhibits 5.1 and 8.1 to this Current Report on Form&nbsp;8-K.</font></p>
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<p style="margin:0in 0in .0001pt;text-indent:24.5pt;"><font size="2" face="Times New Roman" style="font-size:10.0pt;">Under the terms of the Equity Distribution Agreement, the Partnership, on behalf of itself and the Selling Unitholder, may also sell Common Units from time to time to the Sales Agent as principal for its own account at a price to be agreed upon at the time of sale. Any sale of Common Units to the Sales Agent as principal would be pursuant to the terms of a separate terms agreement between the Partnership, on behalf of itself and the Selling Unitholder, and the Sales Agent.</font></p>
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<p style="margin:0in 0in .0001pt;text-indent:29.7pt;"><font size="2" face="Times New Roman" style="font-size:10.0pt;">The foregoing description is a brief summary of the Equity Distribution Agreement and does not purport to be a complete statement of the parties&#146; rights and obligations under the Equity Distribution Agreement. The foregoing description is qualified in its entirety by reference to the full text of the Equity Distribution Agreement, which is filed as Exhibit&nbsp;1.1 to this Current Report on Form&nbsp;8-K and is incorporated by reference herein.</font></p>
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<p style="margin:0in 0in .0001pt;"><b><font size="2" face="Times New Roman" style="font-size:10.0pt;font-weight:bold;">ITEM 9.01.&nbsp; Financial Statements and Exhibits.</font></b></p>
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<p style="margin:0in 0in .0001pt 64.9pt;text-indent:-20.9pt;"><font size="2" face="Times New Roman" style="font-size:10.0pt;">(d)&nbsp;Exhibits.</font></p>
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<p align="center" style="margin:0in 0in .0001pt;text-align:center;"><b><font size="1" face="Times New Roman" style="font-size:8.0pt;font-weight:bold;">Description&nbsp;of&nbsp;Exhibit</font></b></p>    </td>   </tr>
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<p style="margin:0in 0in .0001pt;"><font size="2" face="Times New Roman" style="font-size:10.0pt;">Equity   Distribution Agreement dated as of August&nbsp;7, 2013, among MarkWest Energy   Partners, L.P., MarkWest Energy Operating Company, L.L.C., M&amp;R MWE   Liberty, LLC and Citigroup Global Markets Inc.</font></p>    </td>   </tr>
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<p style="margin:0in 0in .0001pt;"><font size="2" face="Times New Roman" style="font-size:10.0pt;">Opinion   of Vinson&nbsp;&amp; Elkins.</font></p>    </td>   </tr>
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<p style="margin:0in 0in .0001pt;"><font size="2" face="Times New Roman" style="font-size:10.0pt;">Opinion   of Vinson&nbsp;&amp; Elkins, relating to tax matters.</font></p>    </td>   </tr>
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<p style="margin:0in 0in .0001pt;"><font size="2" face="Times New Roman" style="font-size:10.0pt;">Consent   of Vinson&nbsp;&amp; Elkins (included in Exhibit&nbsp;5.1).</font></p>    </td>   </tr>
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<p style="margin:0in 0in .0001pt;"><font size="2" face="Times New Roman" style="font-size:10.0pt;">Consent   of Vinson&nbsp;&amp; Elkins (included in Exhibit&nbsp;8.1).</font></p>    </td>   </tr>  </table>
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<p align="center" style="margin:0in 0in .0001pt;text-align:center;"><b><font size="2" face="Times New Roman" style="font-size:10.0pt;font-weight:bold;">SIGNATURE</font></b></p>
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<p style="margin:0in 0in .0001pt;text-indent:29.7pt;"><font size="2" face="Times New Roman" style="font-size:10.0pt;">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.</font></p>
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<p style="margin:0in 0in .0001pt;"><font size="2" face="Times New Roman" style="font-size:10.0pt;">MARKWEST   ENERGY PARTNERS, L.P.</font></p>    </td>   </tr>
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<p style="margin:0in 0in .0001pt;"><font size="2" face="Times New Roman" style="font-size:1.0pt;">&nbsp;</font></p>    </td>
<td width="4%" valign="top" style="padding:0in 0in 0in 0in;width:4.42%;">
<p style="margin:0in 0in .0001pt;"><font size="2" face="Times New Roman" style="font-size:1.0pt;">&nbsp;</font></p>    </td>
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<p style="margin:0in 0in .0001pt .75in;"><font size="2" face="Times New Roman" style="font-size:10.0pt;">(Registrant)</font></p>    </td>   </tr>
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<p style="margin:0in 0in .0001pt;"><font size="2" face="Times New Roman" style="font-size:10.0pt;">&nbsp;</font></p>    </td>
<td width="4%" valign="top" style="padding:0in 0in 0in 0in;width:4.42%;">
<p style="margin:0in 0in .0001pt;"><font size="2" face="Times New Roman" style="font-size:10.0pt;">&nbsp;</font></p>    </td>
<td width="45%" valign="top" style="padding:0in 0in 0in 0in;width:45.42%;">
<p align="center" style="margin:0in 0in .0001pt;text-align:center;"><font size="2" face="Times New Roman" style="font-size:10.0pt;">&nbsp;</font></p>    </td>   </tr>
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<p style="margin:0in 0in .0001pt;"><font size="2" face="Times New Roman" style="font-size:10.0pt;">&nbsp;</font></p>    </td>
<td width="4%" valign="top" style="padding:0in 0in 0in 0in;width:4.42%;">
<p style="margin:0in 0in .0001pt;"><font size="2" face="Times New Roman" style="font-size:10.0pt;">&nbsp;</font></p>    </td>
<td width="45%" valign="top" style="padding:0in 0in 0in 0in;width:45.42%;">
<p align="center" style="margin:0in 0in .0001pt;text-align:center;"><font size="2" face="Times New Roman" style="font-size:10.0pt;">&nbsp;</font></p>    </td>   </tr>
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<td width="50%" valign="top" style="padding:0in 0in 0in 0in;width:50.16%;">
<p style="margin:0in 0in .0001pt;"><font size="2" face="Times New Roman" style="font-size:1.0pt;">&nbsp;</font></p>    </td>
<td width="4%" valign="top" style="padding:0in 0in 0in 0in;width:4.42%;">
<p style="margin:0in 0in .0001pt;"><font size="2" face="Times New Roman" style="font-size:10.0pt;">By:</font></p>    </td>
<td width="45%" valign="top" style="padding:0in 0in 0in 0in;width:45.42%;">
<p style="margin:0in 0in .0001pt .5in;"><font size="2" face="Times New Roman" style="font-size:10.0pt;">MarkWest   Energy GP, L.L.C.,</font></p>    </td>   </tr>
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<p style="margin:0in 0in .0001pt;"><font size="2" face="Times New Roman" style="font-size:1.0pt;">&nbsp;</font></p>    </td>
<td width="4%" valign="top" style="padding:0in 0in 0in 0in;width:4.42%;">
<p style="margin:0in 0in .0001pt;"><font size="2" face="Times New Roman" style="font-size:1.0pt;">&nbsp;</font></p>    </td>
<td width="45%" valign="top" style="padding:0in 0in 0in 0in;width:45.42%;">
<p style="margin:0in 0in .0001pt .75in;"><font size="2" face="Times New Roman" style="font-size:10.0pt;">Its   General Partner</font></p>    </td>   </tr>
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<p style="margin:0in 0in .0001pt;"><font size="2" face="Times New Roman" style="font-size:10.0pt;">&nbsp;</font></p>    </td>
<td width="4%" valign="top" style="padding:0in 0in 0in 0in;width:4.42%;">
<p style="margin:0in 0in .0001pt;"><font size="2" face="Times New Roman" style="font-size:10.0pt;">&nbsp;</font></p>    </td>
<td width="45%" valign="top" style="padding:0in 0in 0in 0in;width:45.42%;">
<p align="center" style="margin:0in 0in .0001pt;text-align:center;"><font size="2" face="Times New Roman" style="font-size:10.0pt;">&nbsp;</font></p>    </td>   </tr>
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<p style="margin:0in 0in .0001pt;"><font size="2" face="Times New Roman" style="font-size:10.0pt;">Date:&nbsp;&nbsp;August&nbsp;7,   2013</font></p>    </td>
<td width="4%" valign="top" style="padding:0in 0in 0in 0in;width:4.42%;">
<p style="margin:0in 0in .0001pt;"><font size="2" face="Times New Roman" style="font-size:10.0pt;">By:</font></p>    </td>
<td width="45%" valign="top" style="border:none;border-bottom:solid windowtext 1.0pt;padding:0in 0in 0in 0in;width:45.42%;">
<p style="margin:0in 0in .0001pt .65in;"><font size="2" face="Times New Roman" style="font-size:10.0pt;">/s/   NANCY K. BUESE</font></p>    </td>   </tr>
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<td width="50%" valign="top" style="padding:0in 0in 0in 0in;width:50.16%;">
<p style="margin:0in 0in .0001pt;"><font size="2" face="Times New Roman" style="font-size:1.0pt;">&nbsp;</font></p>    </td>
<td width="4%" valign="top" style="padding:0in 0in 0in 0in;width:4.42%;">
<p style="margin:0in 0in .0001pt;"><font size="2" face="Times New Roman" style="font-size:1.0pt;">&nbsp;</font></p>    </td>
<td width="45%" valign="top" style="border:none;padding:0in 0in 0in 0in;width:45.42%;">
<p style="margin:0in 0in .0001pt .85in;"><font size="2" face="Times New Roman" style="font-size:10.0pt;">Nancy   K. Buese</font></p>    </td>   </tr>
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<p style="margin:0in 0in .0001pt;"><i><font size="2" face="Times New Roman" style="font-size:1.0pt;font-style:italic;">&nbsp;</font></i></p>    </td>
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<p style="margin:0in 0in .0001pt;"><i><font size="2" face="Times New Roman" style="font-size:1.0pt;font-style:italic;">&nbsp;</font></i></p>    </td>
<td width="45%" valign="top" style="padding:0in 0in 0in 0in;width:45.42%;">
<p style="margin:0in 0in .0001pt;"><i><font size="2" face="Times New Roman" style="font-size:10.0pt;font-style:italic;">Executive Vice President and Chief Financial Officer</font></i></p>    </td>   </tr>  </table>
<p align="center" style="margin:0in 0in .0001pt;text-align:center;"><font size="2" face="Times New Roman" style="font-size:10.0pt;">&nbsp;</font></p>
<p align="center" style="margin:0in 0in .0001pt;text-align:center;"><font size="2" face="Times New Roman" style="font-size:10.0pt;">3<a name="PB_3_125820_5335"></a></font></p>
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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):
August 7, 2013
MARKWEST ENERGY PARTNERS, L.P.
(Exact name of registrant as specified in its charter)
Delaware
001-31239
27-0005456
(State or other jurisdiction of
incorporation or organization)
(Commission File Number)
(I.R.S. Employer
Identification Number)
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 Agreement.
On August 7, 2013, MarkWest Energy Partners, L.P. (the "Partnership"), MarkWest Energy Operating Company, L.L.C. (the "Operating Company") and M&R MWE Liberty, LLC (the "Selling Unitholder") entered into an Equity Distribution Agreement (the "Equity Distribution Agreement") with Citigroup Global Markets Inc. (the "Sales Agent"). Pursuant to the Equity Distribution Agreement, the Partnership may sell from time to time through the Sales Agent common units representing limited partner interests (the "Common Units") having an aggregate offering price of up to $400,000,000 (the "Primary Offering").  In addition, the Selling Unitholder may sell from time to time through the Sales Agent up to 1,452,415 Common Units (the "Secondary Offering" and, together with the Primary Offering, the "Common Unit Offering").  Such sales under the Secondary Offering will be made concurrently with the sales of the Primary Offering.  The Selling Unitholder has granted the Partnership a stock power (the "Stock Power") relating to all of the Selling Unitholder's Common Units.  Pursuant to this Stock Power, up to 10% of the Common Units disposed of in any Common Unit Offering will be sold on behalf of the Selling Unitholder, and the remaining Common Units disposed of in any such sale will be sold on behalf of the Partnership.  The Common Units to be sold in the Primary Offering are registered under the Securities Act of 1933, as amended (the "Securities Act"), pursuant to a registration statement on Form S-3 (File No. 333-184482).  The Common Units to be sold in the Secondary Offering are registered under the Securities Act pursuant to a registration statement on Form S-3 (File No. 333-186011).  Sales of the Common Units, if any, will be made by means of ordinary brokers' transactions on the New York Stock Exchange at market prices, in block transactions or as otherwise agreed by the Partnership and the Sales Agent.  Legal opinions related to the Common Units are filed as Exhibits 5.1 and 8.1 to this Current Report on Form 8-K.
Under the terms of the Equity Distribution Agreement, the Partnership, on behalf of itself and the Selling Unitholder, may also sell Common Units from time to time to the Sales Agent as principal for its own account at a price to be agreed upon at the time of sale. Any sale of Common Units to the Sales Agent as principal would be pursuant to the terms of a separate terms agreement between the Partnership, on behalf of itself and the Selling Unitholder, and the Sales Agent.
The foregoing description is a brief summary of the Equity Distribution Agreement and does not purport to be a complete statement of the parties' rights and obligations under the Equity Distribution Agreement. The foregoing description is qualified in its entirety by reference to the full text of the Equity Distribution Agreement, which is filed as Exhibit 1.1 to this Current Report on Form 8-K and is incorporated by reference herein.
ITEM 9.01.  Financial Statements and Exhibits.
(d) Exhibits.
Exhibit No.
Description of Exhibit
1.1
Equity   Distribution Agreement dated as of August 7, 2013, among MarkWest Energy   Partners, L.P., MarkWest Energy Operating Company, L.L.C., M&R MWE   Liberty, LLC and Citigroup Global Markets Inc.
5.1
Opinion   of Vinson & Elkins.
8.1
Opinion   of Vinson & Elkins, relating to tax matters.
23.1
Consent   of Vinson & Elkins (included in Exhibit 5.1).
23.2
Consent   of Vinson & Elkins (included in Exhibit 8.1).
2
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.
MARKWEST   ENERGY PARTNERS, L.P.
(Registrant)
By:
MarkWest   Energy GP, L.L.C.,
Its   General Partner
Date:  August 7,   2013
By:
/s/   NANCY K. BUESE
Nancy   K. Buese
Executive Vice President and Chief Financial Officer
3