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Exhibit 10.3

VOTING AGREEMENT

BY AND AMONG

MARKWEST ENERGY PARTNERS L.P.

AND

THE FOX FAMILY HOLDERS

DATED AS OF SEPTEMBER 5, 2007

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TABLE OF CONTENTS

 
   
  PAGE

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ARTICLE 1       GENERAL   1   1.1   Defined Terms   1 ARTICLE 2       VOTING   2
  2.1   Agreement to Vote   2   2.2   No Inconsistent Agreements   3   2.3  
Proxy   3 ARTICLE 3       REPRESENTATIONS AND WARRANTIES   4   3.1  
Representations and Warranties of the Stockholders   4   3.2   Representations
and Warranties of Energy Partners   5 ARTICLE 4       OTHER COVENANTS   5   4.1
  Prohibition on Transfers, Other Actions   5   4.2   Stock Dividends, etc   5  
4.3   No Solicitation   5   4.4   Notice of Acquisitions, Proposals Regarding
Prohibited Transactions   6   4.5   Waiver of Appraisal Rights   6   4.6  
Further Assurances   6   4.7   Stockholder Capacity   6   4.8   Registration
Rights Agreement   7 ARTICLE 5       MISCELLANEOUS   7   5.1   Termination   7  
5.2   No Ownership Interest   7   5.3   Publicity   7   5.4   Notices   7   5.5
  Interpretation   8   5.6   Counterparts   9   5.7   Entire Agreement   9   5.8
  Governing Law; Consent to Jurisdiction; Waiver of Jury Trial   9   5.9  
Amendment; Waiver   10   5.10   Remedies   10   5.11   Severability   10   5.12
  Successors and Assigns; Third Party Beneficiaries   10

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Affiliate   1 Agreement   1 Beneficial Ownership   1 Beneficially Own   2
Beneficially Owned   2 Common Stock   1, 2 control   2 controlled by   2 Covered
Shares   2 Energy Partners   1 Existing Shares   1 Grantees   3 Hydrocarbon   1
Lien   2 Merger Agreement   1 Merger Sub   1 Orders   4 Person   2
Representatives   2 Specified Rights   4 Stockholders   1 Transfer   2 under
common control with   2

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VOTING AGREEMENT

        VOTING AGREEMENT, dated as of September 5, 2007 (this "Agreement"), by
and among MARKWEST ENERGY PARTNERS, L.P., a Delaware limited partnership
("Energy Partners"), and JOHN M. FOX AND MWHC HOLDING, INC., a Colorado
corporation (collectively, the "Stockholders" and, individually, "Stockholder").

W I T N E S S E T H:

        WHEREAS, concurrently with the execution of this Agreement, Energy
Partners, MarkWest Hydrocarbon, Inc., a Delaware corporation ("Hydrocarbon") and
WMEP, L.L.C., a Delaware limited liability company ("Merger Sub") are entering
into an Agreement and Plan of Merger and Redemption, dated as of the date hereof
(as amended, supplemented, restated or otherwise modified from time to time, the
"Merger Agreement") pursuant to which, among other things, Merger Sub will merge
with and into Hydrocarbon and each outstanding share of the common stock, par
value $0.01 per share, of Hydrocarbon (the "Common Stock") will be converted
into the right to receive the merger consideration specified therein;

        WHEREAS, as of the date hereof, each Stockholder is the record and
beneficial owner, in the aggregate, of the number of shares of Common Stock set
forth opposite such Stockholder's name on Schedule I hereto (the "Existing
Shares"), all of which such shares such Stockholder controls the right to vote;

        WHEREAS, as a material inducement to Energy Partners entering into the
Merger Agreement, Energy Partners has required that the Stockholders agree, and
the Stockholders have agreed, to enter into this agreement and abide by the
covenants and obligations with respect to the Covered Shares (as hereinafter
defined) set forth herein; and

        WHEREAS, as a material inducement to the Stockholders entering into this
Agreement, Energy Partners has agreed to enter into a Registration Rights
Agreement on the Closing Date and grant the Stockholders certain registration
rights as provided therein.

        NOW THEREFORE, in consideration of the foregoing and the mutual
representations, warranties, covenants and agreements herein contained, and
intending to be legally bound hereby, the parties hereto agree as follows:

ARTICLE 1

GENERAL

        1.1    Defined Terms.    The following capitalized terms, as used in
this Agreement, shall have the meanings set forth below. Capitalized terms used
but not otherwise defined herein shall have the meanings ascribed thereto in the
Merger Agreement.

        "Affiliate" means, with respect to any Person, any other Person that
directly, or indirectly through one or more intermediaries, controls, is
controlled by or is under common control with, such specified Person. For
purposes of this Agreement, with respect to each Stockholder or other Person,
Affiliate shall not include Hydrocarbon or any Person that is directly or
indirectly, through one or more intermediaries, controlled by Hydrocarbon. For
the avoidance of doubt, no officer or director of Hydrocarbon, the General
Partner, Energy Partners or any of their controlled Affiliates shall be deemed
to be an Affiliate of a Stockholder or other Person by virtue of his, her or its
status as a director or officer of Hydrocarbon, the General Partner, Energy
Partners or any of their controlled Affiliates.

        "Beneficial Ownership" by a Person of any securities includes ownership
by any Person who, directly or indirectly, including through any contract,
arrangement, understanding, relationship or otherwise, has or shares (i) voting
power which includes the power to vote, or to direct the voting of, such
security; and/or (ii) investment power which includes the power to dispose, or
to direct the disposition, of such

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security; and shall otherwise be interpreted in accordance with the term
"beneficial ownership" as defined in Rule 13d-3 adopted by the Securities and
Exchange Commission under the Exchange Act; provided that for purposes of
determining Beneficial Ownership, a Person shall be deemed to be the Beneficial
Owner of any securities which such Person has, at any time during the term of
this Agreement, the right to acquire pursuant to any agreement, arrangement or
understanding or upon the exercise of conversion rights, exchange rights,
warrants or options, or otherwise (irrespective of whether the right to acquire
such securities is exercisable immediately or only after the passage of time,
including the passage of time in excess of 60 days, the satisfaction of any
conditions, the occurrence of any event or any combination of the foregoing).
The terms "Beneficially Own" and "Beneficially Owned" shall have a correlative
meaning. Neither Stockholder shall be deemed to Beneficially Own any of the
shares of Common Stock held by any Person identified on Schedule II hereto (the
"Excluded Shares").

        "Common Stock" means, the common stock, par value $.01 per share, of
Hydrocarbon.

        "control" (including the terms "controlled by" and "under common control
with"), with respect to the relationship between or among two or more Persons,
means the possession, directly or indirectly, of the power to direct or cause
the direction of the affairs or management of a Person, whether through the
ownership of voting securities, as trustee or executor, by contract or any other
means.

        "Covered Shares" means, with respect to a Stockholder, such
Stockholder's Existing Shares, together with any shares of Common Stock or other
voting capital stock of Hydrocarbon and any securities convertible into or
exercisable or exchangeable for shares of Common Stock or other voting capital
stock of Hydrocarbon, in each case that such Stockholder acquires Beneficial
Ownership of on or after the date hereof, but shall not include the Excluded
Shares.

        "Lien" means any mortgage, lien, charge, restriction (including
restrictions on transfer), pledge, security interest, option, right of first
offer or refusal, preemptive right, put or call option, lease or sublease,
claim, right of any third party, covenant, right of way, easement, encroachment
or encumbrance.

        "Person" means any individual, corporation, limited liability company,
limited or general partnership, joint venture, association, joint stock company,
trust, unincorporated organization, government or any agency or political
subdivision thereof or any other entity, or any group comprised of two or more
of the foregoing.

        "Representatives" means the officers, directors, employees, agents,
advisors and Affiliates of a Person.

        "Transfer" means, directly or indirectly, to sell, transfer, assign,
pledge, encumber, hypothecate or similarly dispose of (by merger (including by
conversion into securities or other consideration), by tendering into any tender
or exchange offer, by testamentary disposition, by operation of law or
otherwise), either voluntarily or involuntarily, or to enter into any contract,
option or other arrangement or understanding with respect to the voting of or
sale, transfer, assignment, pledge, encumbrance, hypothecation or similar
disposition of (by merger, by tendering into any tender or exchange offer, by
testamentary disposition, by operation of law or otherwise).

ARTICLE 2

VOTING

        2.1    Agreement to Vote.    Each Stockholder hereby irrevocably and
unconditionally agrees that during the term of this Agreement, at the
Hydrocarbon Meeting and at any other meeting of the stockholders of Hydrocarbon,
however called, including any adjournment or postponement thereof, and

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in connection with any written consent of the stockholders of Hydrocarbon, such
Stockholder shall, in each case to the fullest extent that the Covered Shares
are entitled to vote thereon or consent thereto:

        (a)   appear at each such meeting or otherwise cause its Covered Shares
to be counted as present thereat for purposes of calculating a quorum; and

        (b)   vote (or cause to be voted), in person or by proxy, or deliver (or
cause to be delivered) a written consent covering, all of the Covered Shares
(i) in favor of the adoption of the Redemption Charter Amendment, the Merger
Agreement and the approval of the Merger and any other action reasonably
requested by Energy Partners in furtherance thereof, submitted for the vote or
written consent of stockholders; (ii) against any action or agreement that would
result in a breach of any covenant, representation or warranty or any other
obligation or agreement of Hydrocarbon contained in the Merger Agreement;
(iii) against any Acquisition Proposal; and (iv) against any action, agreement
or transaction that would impede, interfere with, delay, postpone, discourage,
frustrate the purposes of or adversely affect the Merger or the other
transactions contemplated by the Merger Agreement.

        2.2    No Inconsistent Agreements.    Each Stockholder hereby covenants
and agrees that, except for this Agreement, such Stockholder (a) has not entered
into, and shall not enter into at any time while this Agreement remains in
effect, any voting agreement or voting trust with respect to its Covered Shares,
(b) has not granted, and shall not grant at any time while this Agreement
remains in effect, a proxy (except pursuant to Section 2.3 hereof), consent or
power of attorney with respect to its Covered Shares and (c) has not taken and
shall not knowingly take any action that would make any representation or
warranty of such Stockholder contained herein untrue or incorrect or have the
effect of preventing or disabling such Stockholder from performing any of his or
its obligations under this Agreement. Notwithstanding the foregoing and
Section 2.3, if a proxy relating to Hydrocarbon's 2008 annual meeting of
stockholders is mailed to Hydrocarbon stockholders prior to the termination of
this Agreement, each Stockholder may grant a proxy with respect to the voting of
his or its Covered Shares at such meeting, provided that no matter relating to
the Merger or any transaction contemplated in the Merger Agreement is to be
considered at such meeting and that such proxy is not otherwise inconsistent
with this Agreement.

        2.3    Proxy.    In order to secure the obligations set forth herein,
Stockholder hereby irrevocably appoints as his or its proxy and
attorney-in-fact, as the case may be Nancy K. Buese and Andy Schroeder, in their
respective capacities as officers of Energy Partners or the General Partner, and
any individual who shall hereafter succeed to any such officer of Energy
Partners or the General Partner, as the case may be, and any other Person
designated in writing by Energy Partners or the General Partner (collectively,
the "Grantees"), each of them individually, with full power of substitution, to
vote or execute written consents with respect to the Covered Shares in
accordance with Section 2.1 hereof and, in the discretion of the Grantees, with
respect to any proposed postponements or adjournments of any annual or special
meeting of the stockholders of Hydrocarbon at which any of the matters described
in Section 2.1(b) are to be considered; provided that any exercise of this proxy
by such Grantees shall be subject to the approval of such exercise by the
Conflicts Committee. This proxy is coupled with an interest and shall be
irrevocable, except upon termination of this Agreement, and each Stockholder
will take such further action or execute such other instruments as may be
necessary to effectuate the intent of this proxy and hereby revokes any proxy
previously granted by such Stockholder with respect to the Covered Shares.
Energy Partners may terminate this proxy with respect to any Stockholder at any
time at its sole election by written notice provided to such Stockholder.

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ARTICLE 3

REPRESENTATIONS AND WARRANTIES

        3.1    Representations and Warranties of the Stockholders.    Each
Stockholder (except to the extent otherwise provided herein) hereby severally
but not jointly represents and warrants to Energy Partners as follows:

        (a)    Organization; Authorization; Validity of Agreement; Necessary
Action.    Stockholder has the requisite power and authority to execute and
deliver this Agreement, to carry out his or its obligations hereunder and to
consummate the transactions contemplated hereby. The execution and delivery by
Stockholder of this Agreement, the performance by him or it of the obligations
hereunder and the consummation of the transactions contemplated hereby have been
duly and validly authorized by Stockholder and no other actions or proceedings
on the part of Stockholder to authorize the execution and delivery of this
Agreement, the performance by Stockholder of the obligations hereunder or the
consummation of the transactions contemplated hereby. This Agreement has been
duly executed and delivered by Stockholder and, assuming the due authorization,
execution and delivery of this Agreement by Energy Partners, constitutes a
legal, valid and binding agreement of Stockholder, enforceable against him or it
in accordance with its terms, subject to bankruptcy, insolvency, fraudulent
transfer, reorganization, moratorium and similar laws of general applicability
relating to or affecting creditors' rights and to general equitable principles.

        (b)    Ownership.    Stockholder's Existing Shares are, and all of the
Covered Shares owned by Stockholder from the date hereof through and on the
Closing Date will be, Beneficially Owned by Stockholder, except to the extent
such Covered Shares constitute any warrants, options, conversion rights or
similar rights with respect to Common Stock (collectively, "Specified Rights")
that expire after the date hereof. Stockholder has good and marketable title to
Stockholder's Existing Shares, free and clear of any Lien. Except to the extent
Covered Shares constitute Specified Rights that expire after the date hereof and
except as set forth on Schedule III, Stockholder has and will have at all times
through the Closing Date sole voting power (including the right to control such
vote as contemplated herein), sole power of disposition, sole power to issue
instructions with respect to the matters set forth in Article 2 hereof, and sole
power to agree to all of the matters set forth in this Agreement, in each case
with respect to all of Stockholder's Existing Shares and with respect to all of
the Covered Shares owned by Stockholder at all times through the Closing Date
(subject, in the case of Covered Shares underlying Specified Rights acquired
after the date hereof, to the terms of such Specified Rights).

        (c)    No Violation.    Neither the execution and delivery of this
Agreement by Stockholder nor the performance by Stockholder of his or its
obligations under this Agreement will (A) result in a violation or breach of or
conflict with any provisions of, or constitute a default (or an event which,
with notice or lapse of time or both, would constitute a default) under, or
result in the termination, cancellation of, or give rise to a right of purchase
under, or accelerate the performance required by, or result in a right of
termination or acceleration under, or result in the creation of any Lien upon
any of the properties, rights or assets, including but not limited to the
Existing Shares, owned or operated by Stockholder, or result in being declared
void, voidable, or without further binding effect, or otherwise result in a
detriment to Stockholder under any of the terms, conditions or provisions of any
note, bond, mortgage, indenture, deed of trust, license, contract, lease,
agreement or other instrument or obligation of any kind to which Stockholder is
a party or by which Stockholder or any of his or its respective properties,
rights or assets may be bound (B) violate any judgments, decrees, injunctions,
rulings, awards, settlements, stipulations, orders (collectively, "Orders") or
laws applicable to Stockholder or any of his properties, rights or

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assets or, in the case of MWHC Holdings, Inc., result in a violation or breach
of or conflict with its certificate of incorporation or bylaws.

        (d)    Consents and Approvals.    No consent, approval, Order or
authorization of, or registration, declaration or filing with, any governmental
authority is necessary to be obtained or made by Stockholder in connection with
Stockholder's execution, delivery and performance of this Agreement or the
consummation by Stockholder of the transactions contemplated hereby, except
(i) for any reports under Sections 13(d) and 16 of the Exchange Act as may be
required in connection with this Agreement and the transactions contemplated
hereby.

        (e)    Absence of Litigation.    There is no action, litigation or
proceeding pending and no Order of any governmental authority outstanding nor,
to the knowledge of Stockholder, is any such action, litigation, proceeding or
Order threatened, against Stockholder or the Existing Shares which may prevent
or materially delay Stockholder from performing his obligations under this
Agreement or consummating the transactions contemplated hereby on a timely
basis.

        (f)    Reliance by Energy Partners.    Stockholder understands and
acknowledges that Energy Partners is entering into the Merger Agreement in
reliance upon Stockholder's execution and delivery of this Agreement and the
representations and warranties of Stockholder contained herein.

        3.2    Representations and Warranties of Energy Partners.    Energy
Partners hereby represents and warrants to each Stockholder that the execution
and delivery of this Agreement by Energy Partners and the consummation of the
transactions contemplated hereby have been duly authorized by all necessary
action on the part of Energy Partners.

ARTICLE 4

OTHER COVENANTS

        4.1    Prohibition on Transfers, Other Actions.    Stockholder hereby
agrees not to (i) Transfer any of the Covered Shares, Beneficial Ownership
thereof or any other interest therein; (ii) enter into any agreement,
arrangement or understanding with any Person, or take any other action, that
violates or conflicts with or would reasonably be expected to violate or
conflict with, or result in or give rise to a violation of or conflict with,
Stockholder's representations, warranties, covenants and obligations under this
Agreement; (iii) take any action that could restrict or otherwise affect
Stockholder's legal power, authority and right to comply with and perform his or
its covenants and obligations under this Agreement; or (iv) permit the Covered
Shares to become subject to any Lien. Any Transfer in violation of this
provision shall be null and void.

        4.2    Stock Dividends, etc.    In the event of a stock split, stock
dividend or distribution, or any change in the Common Stock by reason of any
split-up, reverse stock split, recapitalization, combination, reclassification,
exchange of shares or the like, the terms "Existing Shares" and "Covered Shares"
shall be deemed to refer to and include such shares as well as all such stock
dividends and distributions and any securities into which or for which any or
all of such shares may be changed or exchanged or which are received in such
transaction.

        4.3    No Solicitation.    Each Stockholder agrees that he or it will
not, and shall use his or its reasonable best efforts to cause his or its
Representatives not to, directly or indirectly through another Person,
(i) solicit, initiate or knowingly encourage, the submission of any Acquisition
Proposal or the making or consummation thereto, (ii) participate in any
discussions or negotiations regarding, or furnish to any person any nonpublic
information about Hydrocarbon in connection with, or otherwise cooperate in any
way with, any Acquisition Proposal, (iii) make or participate in, directly or
indirectly, a "solicitation" of "proxies" (as such terms are used in the rules
of the U.S. Securities and Exchange

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Commission) or powers of attorney or similar rights to vote, or seek to advise
or influence any Person with respect to the voting of, any shares of Common
Stock in connection with any vote or other action on any matter, other than to
recommend that stockholders of Hydrocarbon vote in favor of the adoption of the
Redemption Charter Amendment and the Merger Agreement and as otherwise expressly
provided for in this Agreement, or (iv) agree or publicly propose to do any of
the foregoing. Notwithstanding anything to the contrary in this Section 4.3,
nothing contained in this Agreement shall prohibit a Stockholder from furnishing
any information to, or entering into or participating in discussions or
negotiations with, any person (that is not an Affiliate of such Stockholder)
that makes a written Acquisition Proposal, not in breach of Section 6.6 of he
Merger Agreement, pursuant to which such Stockholder would be treated on an
equivalent basis with Hydrocarbon's other holders of Common Stock if (i) the
Deal Committee determines that such Acquisition Proposal constitutes or is
likely to result in a Superior Proposal and (ii) prior to Stockholder furnishing
such non-public information to such person, Hydrocarbon receives from such
person an executed confidentiality agreement no less restrictive than the
Confidentiality Agreement and furnishes Energy Partners with any such
information that has not previously been furnished. Each Stockholder hereby
represents that, as of the date hereof, such Stockholder is not engaged in any
discussions or negotiations with respect to any Acquisition Proposal and shall
use his or its reasonable best efforts to cause such Stockholder's
Representatives to immediately cease and cause to be terminated all existing
discussions or negotiations with any Person conducted heretofore with respect to
any Acquisition Proposal and request the prompt return or destruction of all
confidential information previously furnished and will take commercially
reasonable steps to inform his or its Representatives of the obligations
undertaken by such Stockholder pursuant to this Agreement, including this
Section 4.3.

        4.4    Notice of Acquisitions, Proposals Regarding Prohibited
Transactions.    Each Stockholder hereby agrees to notify Energy Partners as
promptly as practicable (and in any event within 24 hours after receipt) of
(i) the number of any additional shares of Common Stock or other securities of
Hydrocarbon of which Stockholder acquires Beneficial Ownership on or after the
date hereof, and (ii) any inquiries or proposals which are received by, any
information which is requested from, or any negotiations or discussions which
are sought to be initiated or continued with, Stockholder or any of his or its
Affiliates with respect to any Acquisition Proposal or any other matter referred
to in Section 4.3 (including the material terms thereof and the identity of such
person(s) making such inquiry or proposal, requesting such information or
seeking to initiate or continue such negotiations or discussions, as the case
may be). Each Stockholder will keep Energy Partners informed on a reasonably
current basis of material developments with respect to any such Acquisition
Proposal.

        4.5    Waiver of Appraisal Rights.    To the fullest extent permitted by
applicable law, each Stockholder hereby agrees not to make a written demand or
file a petition for appraisal in respect of its Covered Shares pursuant to
Section 262 of the General Corporation Law of the State of Delaware in
connection with the Merger or the Redemption Charter Amendment.

        4.6    Further Assurances.    From time to time, at Energy Partners'
request and without further consideration, each Stockholder shall execute and
deliver such additional documents and take all such further action as may be
reasonably necessary or advisable to effect the actions and consummate the
transactions contemplated by this Agreement.

        4.7    Stockholder Capacity.    Each Stockholder has entered into this
Agreement solely in the capacity as a Beneficial Owner of Covered Shares.
Notwithstanding anything to the contrary contained in this Agreement: (i) none
of the provisions of this Agreement shall be construed to prohibit, limit or
restrict either Stockholder or any of its Representatives who is an officer or a
member of the Board of Directors of Hydrocarbon or Energy Partners from
exercising his or its fiduciary duties to Hydrocarbon or Energy Partners by
voting or taking any other action whatsoever in his or her capacity as an
officer or director, including with respect to the Merger Agreement and the
transactions contemplated thereby; and (ii) no action taken by Hydrocarbon or
Energy Partners in respect of any Acquisition

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Proposal shall serve as the basis of a claim that a Stockholder is in breach of
its obligations hereunder notwithstanding the fact that such Stockholder or its
Representatives have provided advice or assistance to Hydrocarbon or Energy
Partners in connection therewith.

        4.8    Registration Rights Agreement.    On the Closing Date, each of
Energy Partners and the Stockholders shall execute and deliver the Registration
Rights Agreement, in the form attached hereto as Exhibit A pursuant to which
Energy Partners will agree to grant certain registration rights to the
Stockholders.

ARTICLE 5

MISCELLANEOUS

        5.1    Termination.    This Agreement shall remain in effect until the
earliest to occur of (i) the Effective Time; (ii) the termination of the Merger
Agreement in accordance with its terms; (iii) a Hydrocarbon Change of
Recommendation; (iv) the amendment of the Merger Agreement to decrease the
Merger Consideration or otherwise alter the Merger Agreement in a manner adverse
to the Stockholders unless such amendment has been consented to by each
Stockholder in writing prior to such amendment; or (v) the written agreement of
the Stockholders and Energy Partners to termination of this Agreement. After the
occurrence of such applicable event, this Agreement shall terminate and be of no
further force. Nothing in this Section 5.1 and no termination of this Agreement
shall relieve or otherwise limit any party of liability for any breach of this
Agreement occurring prior to such termination.

        5.2    No Ownership Interest.    Nothing contained in this Agreement
shall be deemed to vest in Energy Partners any direct or indirect ownership or
incidence of ownership of or with respect to any Covered Shares. All rights,
ownership and economic benefit relating to the Covered Shares shall remain
vested in and belong to each Stockholder, and Energy Partners shall have no
authority to direct such Stockholder in the voting or disposition of any of the
Covered Shares, except as otherwise provided herein.

        5.3    Publicity.    Each Stockholder hereby permits Energy Partners and
Hydrocarbon to include and disclose in the Registration Statement, the Joint
Proxy Statement and in such other schedules, certificates, applications,
agreements or documents as such entities reasonably determine to be necessary or
appropriate in connection with the consummation of the Merger and the
transaction contemplated in the Merger Agreement such Stockholder's identity and
ownership of the Covered Shares and the nature of such Stockholder's
commitments, arrangements and understandings pursuant to this Agreement.

        5.4    Notices.    All notices and other communications hereunder shall
be in writing and shall be deemed given when delivered personally or by telecopy
(upon telephonic confirmation of receipt) or on the first Business Day following
the date of dispatch if delivered by a recognized next day courier service. All
notices hereunder shall be delivered as set forth below or pursuant to such
other instructions as may be designated in writing by the party to receive such
notice:

If to Energy Partners, to:

MarkWest Energy GP, L.L.C.
Attn: General Counsel
1515 Arapahoe Street
Tower 2, Suite 700
Denver, Colorado 80202
Fax: (303) 290-8769

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With copies to:

Vinson & Elkins L.L.P.
Attn: Michael J. Swidler
666 Fifth Avenue, 26th Floor
New York, NY 10103
Fax: (212)237-0100

Andrews Kurth LLP
Attn: Bill Cooper
1350 I Street, N.W., Suite 1100
Washington, D.C. 20005
Fax: (202) 662-2739

If to Hydrocarbon, to:

MarkWest Hydrocarbon, Inc.
Attn: General Counsel
1515 Arapahoe Street
Tower 2, Suite 700
Denver, Colorado 80202
Fax: (303) 290-8769

With copies to:

Hogan & Hartson L.L.P.
Attn: George Hagerty
One Tabor Center, Suite 1500
1200 Seventeenth Street
Denver, Colorado 80202
Fax: (303) 899-7333

Akin, Gump, Strauss, Hauer & Feld LLP
Attn: Michael E. Dillard, P.C.
1111 Louisiana Street, 44th Floor
Houston, Texas 77002
Fax: (713) 236-0832

If to a Stockholder, to:

MWHC Holding, Inc.
Attn: John Fox
155 Inverness Drive West, #330
Englewood, Colorado 80112
Fax: (303) 649-2138

With copies to:

Cooley Godward Kronish LLP
Attn: Francis R. Wheeler, Esq.
380 Interlocken Crescent, Suite 900
Broomfield, Colorado 80021
Fax: (720) 566-4499

        5.5    Interpretation.    The words "hereof," "herein" and "hereunder"
and words of similar import when used in this Agreement shall refer to this
Agreement as a whole and not to any particular provision of this Agreement, and
Section references are to this Agreement unless otherwise specified.

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Whenever the words "include," "includes" or "including" are used in this
Agreement, they shall be deemed to be followed by the words "without
limitation." The meanings given to terms defined herein shall be equally
applicable to both the singular and plural forms of such terms. The table of
contents and headings contained in this Agreement are for reference purposes
only and shall not affect in any way the meaning or interpretation of this
Agreement. This Agreement is the product of negotiation by the parties having
the assistance of counsel and other advisers. It is the intention of the parties
that this Agreement not be construed more strictly with regard to one party than
with regard to the others.

        5.6    Counterparts.    This Agreement may be executed by facsimile and
in counterparts, all of which shall be considered one and the same agreement and
shall become effective when counterparts have been signed by each of the parties
and delivered to the other parties, it being understood that all parties need
not sign the same counterpart.

        5.7    Entire Agreement.    This Agreement and, solely to the extent of
the defined terms referenced herein, the Merger Agreement, together with the
schedule annexed hereto, embody the complete agreement and understanding among
the parties hereto with respect to the subject matter hereof and supersede and
preempt any prior understandings, agreements or representations by or among the
parties, written and oral, that may have related to the subject matter hereof in
any way.

        5.8    Governing Law; Consent to Jurisdiction; Waiver of Jury Trial.    

        (a)   This Agreement shall be governed by and construed in accordance
with the laws of the State of Delaware, regardless of the laws that might
otherwise govern under applicable principles of conflicts of laws thereof. The
parties agree that irreparable damage would occur and that the parties would not
have any adequate remedy at law in the event that any of the provisions of this
Agreement were not performed in accordance with their specific terms or were
otherwise breached. It is accordingly agreed that the parties shall be entitled
to an injunction or injunctions to prevent breaches of this Agreement and to
enforce specifically the terms and provisions of this Agreement in the Court of
Chancery of the State of Delaware (and any appellate court of the State of
Delaware) and the Federal courts of the United States of America located in the
State of Delaware, this being in addition to any other remedy to which they are
entitled at law or in equity. In addition, each of the parties hereto
(i) consents to submit itself to the personal jurisdiction of the Court of
Chancery of the State of Delaware (and any appellate court of the State of
Delaware) and the Federal courts of the United States of America located in the
State of Delaware in the event any dispute arises out of this Agreement or the
transactions contemplated by this Agreement, (ii) agrees that it will not
attempt to deny or defeat such personal jurisdiction by motion or other request
for leave from any such court and (iii) agrees that it will not bring any action
relating to this Agreement or the transactions contemplated by this Agreement in
any court other than the Court of Chancery of the State of Delaware or a Federal
court of the United States of America located in the State of Delaware.

        (b)   EACH PARTY HERETO HEREBY WAIVES, TO THE FULLEST EXTENT PERMITTED
BY APPLICABLE LAW, ANY RIGHT IT MAY HAVE TO A TRIAL BY JURY IN RESPECT OF ANY
SUIT, ACTION OR OTHER PROCEEDING ARISING OUT OF THIS AGREEMENT OR THE
TRANSACTIONS CONTEMPLATED HEREBY. EACH PARTY HERETO (A) CERTIFIES THAT NO
REPRESENTATIVE, AGENT OR ATTORNEY OF ANY OTHER PARTY HAS REPRESENTED, EXPRESSLY
OR OTHERWISE, THAT SUCH PARTY WOULD NOT, IN THE EVENT OF ANY ACTION, SUIT OR
PROCEEDING, SEEK TO ENFORCE THE FOREGOING WAIVER AND (B) ACKNOWLEDGES THAT IT
AND THE OTHER PARTIES HERETO HAVE BEEN INDUCED TO ENTER INTO THIS AGREEMENT, BY,
AMONG OTHER THINGS, THE MUTUAL WAIVER AND CERTIFICATIONS IN THIS SECTION 5.9.

9

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        5.9    Amendment; Waiver.    This Agreement may not be amended except by
an instrument in writing signed by Energy Partners and each Stockholder. Each
party may waive any right of such party hereunder by an instrument in writing
signed by such party and delivered to Energy Partners and the Stockholders. No
amendment or waiver shall be effective unless, (a) in the case of Energy
Partners, the Conflicts Committee shall have approved such amendment or waiver,
and (b) the Deal Committee, on behalf of Hydrocarbon, shall have given its
written consent, such consent not to be unreasonably withheld.

        5.10    Remedies.    

        (a)   Each party hereto acknowledges that monetary damages would not be
an adequate remedy in the event that any covenant or agreement in this Agreement
is not performed in accordance with its terms, and it is therefore agreed that,
in addition to and without limiting any other remedy or right it may have, the
non-breaching party will have the right to an injunction, temporary restraining
order or other equitable relief in any court of competent jurisdiction enjoining
any such breach and enforcing specifically the terms and provisions hereof. Each
party hereto agrees not to oppose the granting of such relief in the event a
court determines that such a breach has occurred, and to waive any requirement
for the securing or posting of any bond in connection with such remedy.

        (b)   All rights, powers and remedies provided under this Agreement or
otherwise available in respect hereof at law or in equity shall be cumulative
and not alternative, and the exercise or beginning of the exercise of any
thereof by any party shall not preclude the simultaneous or later exercise of
any other such right, power or remedy by such party.

        5.11    Severability.    Any term or provision of this Agreement which
is determined by a court of competent jurisdiction to be invalid or
unenforceable in any jurisdiction shall, as to that jurisdiction, be ineffective
to the extent of such invalidity or unenforceability without rendering invalid
or unenforceable the remaining terms and provisions of this Agreement or
affecting the validity or enforceability of any of the terms or provisions of
this Agreement in any other jurisdiction, and if any provision of this Agreement
is determined to be so broad as to be unenforceable, the provision shall be
interpreted to be only so broad as is enforceable, in all cases so long as
neither the economic nor legal substance of the transactions contemplated hereby
is affected in any manner adverse to any party or its equityholders. Upon any
such determination, the parties shall negotiate in good faith in an effort to
agree upon a suitable and equitable substitute provision to effect the original
intent of the parties as closely as possible and to the end that the
transactions contemplated hereby shall be fulfilled to the maximum extent
possible.

        5.12    Successors and Assigns; Third Party Beneficiaries.    Neither
this Agreement nor any of the rights or obligations of any party under this
Agreement shall be assigned, in whole or in part (by operation of law or
otherwise), by any party without the prior written consent of the other parties
hereto. Subject to the foregoing, this Agreement shall bind and inure to the
benefit of and be enforceable by the parties hereto and their respective
successors and permitted assigns. Nothing in this Agreement, express or implied,
is intended to confer on any Person other than (a) the parties hereto,
(b) Hydrocarbon, solely to the extent specified in the next succeeding sentence,
or (c) the parties respective successors and permitted assigns any rights,
remedies, obligations or liabilities under or by reason of this Agreement. The
parties hereto agree that Hydrocarbon shall be an intended third party
beneficiary of this Agreement solely to the extent of its right to consent to
amendments to or waivers under this Agreement as specified in Section 5.9.

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        IN WITNESS WHEREOF, the parties hereto have caused this Agreement to be
signed (where applicable, by their respective officers or other authorized
Person thereunto duly authorized) as of the date first written above.

    MARKWEST ENERGY PARTNERS, L.P.:
 
 
BY:
 
MARKWEST ENERGY GP, L.L.C.,
its General Partner
 
 
By:
 
 
 
 
/s/  NANCY K. BUESE      

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Name
 
 
Chief Financial Officer

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Title
 
 
STOCKHOLDERS:
 
 
/s/  JOHN M. FOX      

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John M. Fox
 
 
MWHC HOLDINGS, INC.
 
 
By:
 
 
 
 
/s/  JOHN M. FOX      

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John M. Fox
 
 

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Title

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Schedule I

STOCKHOLDER INFORMATION

Name

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  Existing Shares Beneficially Owned

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MWHC Holding, Inc.(1)   4,842,387
John M. Fox
 
at least 331,852

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(1)Shares Beneficially Owned by MWHC Holding, Inc. are also Beneficially Owned
by John M. Fox.

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Schedule II

EXCLUDED SHARES

Name

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  Excluded Shares Beneficially Owned

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Fox Family Foundation   118,000
Bode Blanco, L.L.C.
 
1,452
Brian T. Crabtree Trust
 
81,250
MaggieGeorge Foundation
 
121,035

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Schedule III

EXCEPTIONS TO SOLE BENEFICIAL OWNERSHIP

        Of the at least 331,852 Existing Shares Beneficially Owned by John M.
Fox, an aggregate of at least 231,830 are owned as joint tenants with his wife,
Marcella F. Fox.

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EXHIBIT A

REGISTRATION RIGHTS AGREEMENT

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QuickLinks

VOTING AGREEMENT BY AND AMONG MARKWEST ENERGY PARTNERS L.P. AND THE FOX FAMILY
HOLDERS DATED AS OF SEPTEMBER 5, 2007
ARTICLE 1 GENERAL
ARTICLE 2 VOTING
ARTICLE 3 REPRESENTATIONS AND WARRANTIES
ARTICLE 4 OTHER COVENANTS
ARTICLE 5 MISCELLANEOUS
Schedule I
STOCKHOLDER INFORMATION
Schedule II
EXCLUDED SHARES
Schedule III
EXCEPTIONS TO SOLE BENEFICIAL OWNERSHIP
EXHIBIT A REGISTRATION RIGHTS AGREEMENT