Exhibit
10.35  SECOND AMENDMENT TO GAS PROCESSING AGREEMENT (Kenova, Boldman and Cobb Plants)  This Second Amendment to Gas
Processing Agreement (Kenova, Boldman and Cobb Plants) is made and entered into
this 26th day of December, 2007, by and between MarkWest
Energy Appalachia, L.L.C.,a Delaware limited partnership (MEA),
and MarkWest Hydrocarbon, Inc., a
Delaware corporation (MarkWest).  MEA
and MarkWest may be referred to individually as Party, or collectively as Parties.  This document is referred to herin as this Second
Amendment.  RECITALS:  A.            MEA
and MarkWest are parties to that certain Gas Processing Agreement (Kenova,
Boldman and Cobb Plants) dated as of May 24, 2002, and amended by that
certain Amendment to Fractionation, Storage and Loading Agreement (Siloam) and
to Gas Processing Agreement (Kenova, Boldman and Cobb Plants) dated November 24,
2003 (the Gas Processing Agreement as so amended is referred to herein as the Processing
Agreement).  B.            MEA
desires to fund certain 2007/2008 capital projects involving upgrades and/or
expansions of MEAs Kenova Plant, Cobb Plant and Boldman Plant as described in
the October 25, 2007 MarkWest Energy GP, L.L.C. Board of Directors meeting
(collectively referred to herein as the Appalachia Expansion Projects), which
projects would in turn, at no capital cost to MarkWest, permit MarkWest to
derive additional frac spread income for NGLs processed by MEAand marketed by the MarkWest. 
In consideration thereof, MEA desires MarkWest to agreeto an increase
of $0.08 per inlet Mcf in the existing Processing Fee under the Processing
Agreement, such increase to commence upon the substantial completion of the
Kenova Plant Upgrade Project portion of theAppalachia
Expansion Projects (presently scheduled for completion second quarter of 2008).  C.            MarkWest
is agreeable to the foregoing provided thatin the event that
all three plant expansions projects (Kenova, Boldman and Cobb Plants) are not
completed and operational by March 31, 2009, the Company and MEA shall
meet to arrive at an appropriate and equitably proportional modification to the
Processing Fee.  Now therefore, in consideration of the mutual covenants and
agreements, the parties agree as follows:  1.             Section 4.A.,
ii, Fees and Consideration, of the Processing Agreement is modified to
provide thatupon the substantial completion and commencement of
operations of the Kenova Plant Upgrade Project portion ofAppalachia Expansion Projects (presently scheduled for
completion second quarter of 2008), the then current dollar multiplier of the
Processing Fee shall be increased by$0.08 per inlet Mcf, provided thatin the event that all three plant
expansions projects (Kenova, Boldman and Cobb Plants) of the Appalachia
Expansion Projects are not substantially completed and operational by March 31,
2009, MarkWest and MEA shall meet to arrive at an appropriate and equitably
proportional modification to the Processing Fee.


2.             Section 6,
Notices, of the Processing Agreement is modified to provide the
following new addresses:  MarkWest:Address
  1515 Arapahoe StreetTower 2, Suite 700Denver, CO 80202Attn: Law DepartmentFax: (303) 925-9308MEA:Address:
  1515 Arapahoe StreetTower 2, Suite 700Denver, CO 80202Attn: Law DepartmentFax: (303) 925-9308  3.             As
amended by this Second Amendment, all terms and conditions of the Processing
Agreement shall remain in full force and effect.  IN WITNESS WHEREOF, the Parties
have executed this Agreement on the date first set forth above.  MARKWEST HYDROCARBON, INC.By:
  /s/ JOHN C. MOLLENKOPFTitle:
  SVP & COOMARKWEST ENERGY APPALACHIA,
  L.L.C.By:
  /s/ RANDY S. NICKERSONTitle:
  CCO  2