<PAGE>

                                                                   Exhibit 10.64

                              EMPLOYMENT AGREEMENT

      This Employment Agreement is entered into effective as of August 28, 2003,
by and among THE UNIMARK GROUP, INC., a Texas corporation (the "Company"), and
RICARDO ARTURO HERRERA BARRE, an individual (the "Employee" or "Mr. Herrera").

      WHEREAS, the Company wishes to continue to employ Employee and Employee
wishes to continue in the employ of the Company for the period and on the terms
and conditions set forth herein;

      NOW, THEREFORE, in consideration of the foregoing promises and the
covenants, conditions, representations and warranties contained herein, the
parties hereto agree as follows:

      1.    Employment. Employee shall perform services for the Company in the
position of President and Chief Executive Officer, and Employee shall report
only to and perform such duties and responsibilities as may be assigned from
time to time by the Company's Board of Directors (the "Board") consistent with
Employee's position. From time to time, the Company may change Employee's title
as it may determine appropriate in light of the Company's current structure and
staffing provided that the title shall properly reflect Employee's status as an
executive. During the term of his employment with the Company, Employee will
devote his best efforts and substantially all of his business time and attention
(except for his attendance to three other boards where he is a current
Director), vacation periods as set forth below and reasonable periods of illness
or other incapacities permitted by the Company's general employment policies) to
the business of the Company.

      2.    Term of Agreement. The term of this Agreement shall commence as of
August 28, 2003 (the "Effective Date"), and shall continue for a one (1) year
period (the "Term") unless Employee's services are terminated pursuant to the
provisions of Section 7 hereof. Unless the Company gives Employee written notice
not less than thirty (30) days before the anniversary date of this Agreement
that the Company does not desire to renew this Agreement, at the end of the
Term, if Employee is still employed by the Company pursuant to this Agreement,
this Agreement shall be automatically extended for an additional one (1) year
period, and all of the terms and conditions of this Agreement shall thereafter
apply during such extended term.

      3.    Location of Services. Since the Company has offices, production
facilities and agricultural assets in several places throughout Mexico and the
United States, Employee may elect to be based in Mexico City and in such case
would take all measures that are required to ensure that all his functions and
objectives are properly accomplished. It is expressly acknowledged and agreed
that Employee will devote enough time in each of the Company's locations to
discharge his duties and services to be performed under this Agreement. Employee
acknowledges and agrees that Employee's job responsibilities may require
extensive travel, and Employee agrees to travel as necessary to discharge those
duties.

      4.    Code of Conduct, Standard Terms; Proprietary Information. The
employment relationship between the parties shall also be governed by the
Company's Code of Conduct, general employment policies and practices of the
Company, including those relating to compliance with the Sarbanes-Oxley Act,
protection of confidential information and assignment of inventions, except that
when the terms of this Agreement differ from or are in conflict with the
Company's general employment policies or practices, this Agreement shall
control.

      5.    Compensation.

            5.1   Base Compensation. Effective as of the Effective Date, the
Company shall pay to Employee at the annual rate of $ 239,700 (Two hundred
thirty nine thousand seven hundred dollars), less taxes and other usual
deductions. After Fiscal 2003, Employee's base compensation shall be reviewed by
the Board annually, and the Board may, but is not required to, change such base
compensation, only to adjust for US

<PAGE>

inflation. Employee's base compensation shall be payable monthly by one of the
Company's Mexican subsidiaries to an entity designated by the Employee.

      Performance Bonus. On the first anniversary date of this Agreement,
Employee shall be entitled to receive a performance bonus, if any, after review
of performance objectives established by Employee and members of the Company's
compensation committee, in such amount as the Board in its sole discretion shall
deem appropriate.

      6.    Benefits.

            6.1   Stock Options. In accordance with the terms of The UniMark
Group, Inc. Employee Stock Option Plan, the Company agrees to issue to Employee
options to purchase an aggregate of 100,000 shares of common stock of the
Company at $0.75 per share (the "Options"). The Options will vest and become
exercisable ratably over a period of four years (25% per year); provided that
the Options will vest and become immediately exercisable in the event of a
Change in Control. As used herein, a "Change in Control" shall occur if the
equity ownership of M&M Nominee, LLC, and its affiliates, be reduced to below
30% of the Company's equity ownership, calculated on a fully-diluted basis.

            6.2   Business Expenses. The Company shall reimburse Employee for
all ordinary and necessary expenses incurred by Employee, including, but not
limited to, business travel, cell phone, and other disbursements in the
performance of Employee's duties for the Company upon presentation within the
time period specified by the Company of an itemized statement of all expenses
incurred showing the date, nature, recipient, purpose and amount of each item,
subject to prior approval of the Company as required of executive employees.

            6.3   Company Car. During the term of this Agreement, the Company
agrees to make available to Employee a 2003 Honda Pilot (the "Car"). It is
agreed that Employee would be able to purchase the Car from the Company at book
value on or before the expiration of this Agreement.

            6.4   Termination of Benefits. All unvested benefits provided under
this Section 6 shall terminate concurrently with termination of Employee's
employment hereunder for any reason whatsoever. Nothing herein shall vest any
rights in any profit sharing or bonus plans, general expense or automotive
reimbursements, and similar fringe benefits that the Company may provide, if
any, beyond the date on which Employee's employment is terminated for any
reason.

7.    Termination of Employment.

            7.1   Termination for Cause. Notwithstanding any other provision of
this Agreement, Employee's employment with the Company may be terminated for
cause at any time by the Company, upon reasonable notice to Employee. For the
purposes of this Agreement, "cause" shall mean (a) any failure to perform
pursuant to the terms of this Agreement and such performance failure is not
corrected within ten (10) days after written notice by the Company or the Board
to Employee thereof; or (b) misconduct, including, but not limited, to: (i)
conviction of a crime or entry of a plea of nolo contendere with regard to a
crime involving moral turpitude or dishonesty, (ii) any breach of the
Confidentiality Agreement or the Company's Code of Conduct, (iii) Employee's
insubordination or refusal to comply with any reasonable request of the Board
relating to the scope or performance of Employee's duties, (iv) conduct that in
the good faith and reasonable determination of the Board demonstrates Employee's
unfitness to serve or (v) in the good faith and reasonable determination of the
Board, the Board determines that Employee's continued employment hereunder is
not in the best interest of the Company.

            7.2   Termination Without Cause. The Company may terminate
Employee's employment under this Agreement without cause at any time upon
written notice to Employee.

            7.3   Termination for Death or Disability. Employment hereunder
shall automatically terminate upon Employee's death or disability. Disability,
for purposes of this Agreement, shall mean a physical

<PAGE>

or mental disability that interferes with Employee's ability to perform duties
pursuant to this Agreement for a continuous period of six (6) months or more.

      8.    Post-Termination Compensation.

            8.1   Termination By the Company for Cause. Notwithstanding any
other provision of this Agreement to the contrary, if Employee's employment is
terminated for cause pursuant to Section 7.1, the Company shall make no further
salary payments except those earned prior to the date of termination and shall
make no further bonus payments.

            8.2   Termination By the Company Without Cause. If the Company
terminates this Agreement without cause as defined in Section 7.2 hereof, then
the Company shall pay Employee severance equal to Employee's base compensation
as determined pursuant to Section 5.1 for three (3) months plus 20 days for each
additional one (1) year term of this Agreement completed by Employee. The
Company shall make no further bonus payments.

            8.3   Termination By Employee's Death or Disability. If employment
shall terminate by reason of Employee's death or disability, the Company shall
pay Employee or Employee's estate severance equal to three (3) months' base
compensation, payable in a lump sum. The Company shall make no further bonus
payments.

      9.    Noncompetition. Until one (1) year after termination of Employee's
employment with the Company, Employee shall not (a) either directly or
indirectly, carry on, engage in or have any interest in any business that
competes with the Company, excepting ownership by Employee of no more than one
percent (1%) of the publicly traded common stock of any corporation, (b) without
the express written consent of the Company, accept employment with, or in any
other manner agree to provide, for compensation, services for any other person
or entity that competes directly or indirectly with the Company, or (c)
materially disrupt, damage, impair or interfere with the business of the
Company, whether by way of interfering with or soliciting its employees,
disrupting its relationship with customers, agents, representatives or vendors,
or otherwise.

      10.   Arbitration. Any and all disputes or controversies, whether of law
or fact of any nature whatsoever, arising from or respecting this Agreement
shall be decided by arbitration by the Judicial Arbitration Mediation Services,
Inc. ("JAMS") in accordance with the rules and regulations of JAMS, or by any
other body mutually agreed upon by the parties. Pre-arbitration discovery shall
be permitted at the request of any party under appropriate protection for
proprietary and confidential business information.

            Before filing a demand for arbitration, a party must send the other
parties written notice identifying the matter in dispute and invoking the
procedures in this paragraph. Such written notice shall be sent promptly after
the party knew or reasonably should have known of an alleged violation of this
Agreement. Within fifteen (15) days after such written notice is given, one or
more principals of each party shall meet at a mutually agreeable location in
Dallas, Texas, for the purpose of determining whether they can resolve the
dispute themselves by written agreement. If the parties fail to resolve the
dispute by written agreement within the fifteen-day (15-day) period, the
complaining party may then initiate the arbitration process by filing a demand
with JAMS or such other body as the parties may agree upon. Nothing in this
paragraph shall prevent a party from seeking temporary equitable relief from
JAMS or such other body as the parties may mutually agree upon during the
fifteen-day (15-day) period if necessary to prevent irreparable harm.

            The arbitrators shall be selected as follows: the Company and
Employee shall each select one (1) independent, qualified arbitrator and the two
(2) arbitrators so selected shall select the third arbitrator. Any party may
disqualify any individual arbitrator who is a present or past employee, owner,
officer, director, relative or consultant to any party hereto or a competing
organization.

            Arbitration shall take place in Dallas, Texas, or any other location
mutually agreeable to the parties. At the request of any party, arbitration
proceedings will be conducted in the utmost secrecy and, in such case, all
documents, testimony and records shall be received, heard and maintained by the
arbitrators in secrecy

<PAGE>

under seal, available for inspection only by the Company or Employee, their
respective attorneys, and their respective experts, consultants or witnesses who
shall agree, in advance and in writing, to receive all such information
confidentially and to maintain such information in secrecy, and make no use of
such information except for the purposes of the arbitration, until such
information shall become generally known.

            The arbitrators, who shall act by majority vote, shall be able to
decree any and all relief of an equitable nature, including but not limited to
such relief as a temporary restraining order, a temporary injunction, or a
permanent injunction, and shall also be able to award damages, with or without
an accounting and costs. The decree or judgment of an award rendered by the
arbitrators may be entered in any court having jurisdiction over the parties.

            Reasonable notice of the time and place of arbitration shall be
given to persons other than the parties, if such notice is required by law, in
which case such persons or their authorized representatives shall have the right
to attend or participate in the arbitration hearing in such manner as the law
shall require.

            If any action is necessary to enforce or interpret the terms of this
Agreement, the prevailing party shall be entitled to reasonable attorneys' fees,
costs, and necessary disbursements in addition to any other relief to which that
party may be entitled.

      11.   Power and Authority. Each party executing this Agreement hereby
covenants, represents and warrants that he or she has full power and authority
to execute this Agreement, that no other consents or approvals of any other
third parties are required or necessary for this Agreement to be so binding
(except as otherwise herein expressly stated) and that this Agreement shall be
fully enforceable in accordance with its terms.

      12.   Heirs, Administrators and Successors. Except as otherwise provided
herein, this Agreement shall inure to the benefit of and be binding upon, the
heirs, administrators and successors of each of the parties hereto.

      13.   Nonassignability. The Company may assign the benefit of this
Agreement to any successor in interest that results from a merger,
reorganization or acquisition. Otherwise, no party to this Agreement may assign
any right hereunder or delegate any duty hereunder without the written consent
of the other party affected by such assignment or delegation.

      14.   No Oral Modification. This Agreement may only be changed or modified
and any provisions hereof may only be waived in or by a writing signed by a
party against whom enforcement of any waiver, change or modification is sought.

      15.   Governing Law. This Agreement shall be deemed to be a contract made
under, and shall be construed in accordance with, the laws of the State of
Texas.

      16.   Severability. If any portion of this Agreement shall be held
illegal, unenforceable, void or voidable by any court, each of the remaining
terms hereof shall nevertheless remain in full force and effect as a separate
contract.

      17.   Right of Setoff. Whenever the Company owes Employee any amounts of
money by virtue of this Agreement or otherwise, the Company shall be entitled to
setoff against any such sums due to Employee the amount of any claims that the
Company has against Employee. This right to setoff shall also apply to amounts
due on the date of termination.

      18.   Counterparts. This Agreement may be executed in one or more
counterparts, each of which shall be deemed an original, but all of which
together shall constitute one and the same instrument.

<PAGE>

      19.   Entire Agreement. This Agreement constitutes the entire agreement
and understanding of the parties pertaining to the matters set forth herein, and
all prior agreements, understandings or representations are hereby terminated
and canceled in their entirely and are of no further force and effect.

      IN WITNESS WHEREOF, the parties hereto have executed this Agreement as of
the date first above written.

                                 COMPANY:

                                 THE UNIMARK GROUP, INC.
                                 A Texas Corporation

                                 By: /s/ David E. Ziegler
                                 ------------------------
                                 David Ziegler, Chief Financial Officer

                                 EMPLOYEE:

                                 By: /s/ Ricardo Arturo Herrera Barre
                                 ------------------------------------
                                 Ricardo Arturo Herrera Barre, President and CEO