Markwest Hydrocarbon (MWP) 10-Q 2006 Q1 Quarterly report

Exhibit 32.1

CERTIFICATION PURSUANT TO
18 U.S.C. SECTION 1350,
AS ADOPTED PURSUANT TO
SECTION 906 OF THE SARBANES-OXLEY ACT OF 2002

In connection with the Quarterly Report of MarkWest Hydrocarbon, Inc. (the “Company”), on Form 10-Q for the period ending March 31, 2006, as filed with the Securities and Exchange Commission on the date hereof (the “Report”), I, Frank M. Semple, Chief Executive Officer of the Company, certify, to the best of my knowledge, pursuant to 18 U.S.C. §1350, as adopted pursuant to §906 of the Sarbanes-Oxley Act of 2002, that:

(1) The Report fully complies with the requirements of section 13(a) or 15(d) of the Securities Exchange Act of 1934; and

(2) The information contained in the Report fairly presents, in all material respects, the financial condition and results of operations of the Company.

/s/ FRANK M. SEMPLE

Frank M. Semple

President & Chief Executive Officer

May 8, 2006