<PAGE>

                                                                  EXHIBIT 10 (s)

                                THIRD AMENDMENT

This Third Amendment (the "Amendment") is made and entered into as of the 22/nd/
day of October, 1999, by and between EOP- SUMMIT LIMITED PARTNERSHIP, a Delaware
limited partnership ("Lessor") and Summit Bancshares, Inc. ("Lessee").

                                  WITNESSETH

A.  WHEREAS, Lessor and Lessee are parties to that certain lease dated February,
1992 for space currently containing approximately 4978 rentable square feet (the
"Original Premises") described as Suite No(s). 604 on the sixth (6th) floor of
the buildings commonly known as Summit Office Park and the address of which is
1200-1300 Summit Ave., Fort Worth, TX (the "Building"), which lease has been
previously amended by First Amendment dated May 3, 1994 (the "First Amendment")
and Second Amendment dated July 8, 1998 (the "Second Amendment") which, among
other things, extended the term of the lease (the First Amendment, Second
Amendment and lease are collectively referred to herein as the "Lease"); and

B.  WHEREAS, Lessee has requested that additional space consisting of
approximately 1,829 rentable square feet on the sixth (6th) floor of the 1300
Summit Building shown on Exhibit A hereto ("Expansion Space") be added to the
Premises; and

C.  WHEREAS, the Lease Term shall expire on February 15, 2000 ("Prior
Termination Date") and the parties desire to extend the term of the lease
subject to the terms and conditions hereinafter set forth; and

NOW, THEREFORE, in consideration of the mutual covenants and agreements herein
contained and other good and valuable consideration the receipt and sufficiency
of which are hereby acknowledged, Lessor and Lessee agree as follows:

     I.   EXPANSION AND EFFECTIVE DATE. Effective as of the Expansion Effective
          ----------------------------
          Date (as hereinafter defined), the Premises as defined in the Lease,
          is hereby increased from 4,978 rentable square feet on the sixth (6th)
          floor to 6,807 rentable square feet on the sixth (6th) floor by the
          addition of the Expansion Space, and from and after the Expansion
          Effective Date, the Original Premises and the Expansion Space,
          collectively, shall be deemed the Premises, as defined in the Lease.
          The Lease Term for the Expansion Space shall commence on the Expansion
          Effective Date and end on the Extended Termination Date. The Expansion
          Space is subject to all the terms and conditions of the Lease except
          as expressly modified herein and except that Lessee shall not be
          entitled to receive any allowance, abatements or other financial
          concessions granted with respect to the Original Premises unless such
          concessions are expressly provided for herein with respect to the
          Expansion Space.

                                       1
<PAGE>

          A.   The Expansion Effective Date shall be the later to occur of (i)
               December 1, 1999 ("Target Expansion Effective Date"), and (ii)
               the date upon which Lessor's improvement work in the Expansion
               Space has been substantially completed; provided, however, that
               if Lessor shall be delayed in substantially completing the
               Lessor's work in the Expansion Space as a result of the
               occurrence of any of the following (a "Delay"):

               1.   Lessee's failure to furnish information or to respond to any
                    request by Lessor for any approval or information within any
                    time period prescribed or, if no time period is prescribed,
                    then within two (2) Business Days of such request; or

               2.   Lessee's insistence on materials, finishes or installations
                    that have long lead times after having first been informed
                    by Lessor that such materials, finishes or installations
                    will cause a Delay; or

               3.   Changes in any plans and specifications; or

               4.   The performance by a person or entity employed by Lessee in
                    the completion of any work (all such work and such persons
                    or entities being subject to the prior approval of Lessor);
                    or

               5.   Any request by Lessee that Lessor delay the completion of
                    any of the Lessor's work; or

               6.   Any breach or default by Lessee in the performance of
                    Lessee's obligations under this Amendment or the Lease; or

               7.   Any delay resulting from Lessee's having taken possession of
                    the Expansion Space for any reason prior to substantial
                    completion of the Lessor's work; or

               8.   Any other delay chargeable to Lessee, its agents, employees
                    or independent contractors;

               then, for the purposes of determining the Expansion Effective
               Date, the date of substantial completion shall be deemed to be
               the day that said Lessor's work would have been substantially
               completed absent any such Delay(s). The Expansion Space shall be
               deemed to be substantially completed on the date that Lessor
               reasonably determines that all Lessor's work has been performed
               (or would have been performed absent of any Delays), other than
               any details of construction, mechanical adjustment or any other
               matter, the noncompletion of which does not materially interfere
               with Lessee's use of the Expansion Space. The adjustment of the
               Expansion Effective date and, accordingly, the postponement of
               Lessee's obligation to pay Rent on the

                                       2
<PAGE>

               Expansion Space, shall be Lessee's sole remedy and shall
               constitute full settlement of all claims that Lessee might
               otherwise have against Lessor by reason of the Expansion Space
               not being ready for occupancy by Lessee on the Target Expansion
               Effective Date.

          B.   In addition to the postponement, if any, of the Expansion
               Effective Date as a result of the applicability of Paragraph I.A.
               of this Amendment, the Expansion Effective Date shall be delayed
               to the extent that Lessor fails to deliver possession of the
               Expansion Space for any other reason (other than Delays by
               Lessee), including but not limited to, holding over by prior
               occupants. Any such delay in the Expansion Effective Date shall
               not subject Lessor to any liability for any loss or damage
               resulting therefrom. If the Expansion Effective Date is delayed,
               the Termination Date under the Lease shall not be similarly
               extended.

     II.  EXTENSION. The Lease Term is hereby extended for a period of
          ---------
          fifty-eight (58) months and thirteen (13) days and shall expire on
          December 31, 2004 ("Extended Termination Date"), unless sooner
          terminated in accordance with the terms of the Lease. That portion of
          the Lease Term commencing the day immediately following the Prior
          Termination Date ("Extension Date") and ending on the Extended
          Termination Date shall be referred to herein as the "Extended Term".

     III. ORIGINAL PREMISES BASE MONTHLY RENT.
          -----------------------------------

          A.   Original Premises up to the Prior Termination Date. The Base
               Monthly Rent, Additional Rent and all other charges under the
               Lease shall be payable as provided therein with respect to the
               Original Premises through and including the day prior to the
               Prior Termination Date.

          B.   Original Premises From and After Extension Date. As of the
               Extension Date, the schedule of monthly installments of Base
               Monthly Rent payable with respect to the Original Premises from
               and after the Extension Date is the following:

               Lessee shall pay Lessor the sum of Three Hundred Twenty-Seven
               Thousand Four Hundred Fourteen and 62/100's Dollars ($327,414.62)
               in fifty-nine (59) installments as follows:

               1.   One (1) installment of $2,600.12 payable for the period from
                    February 16, 2000 to February 28, 2000.

               2.   Fifty-Eight (58) equal installments of $5,600.25 each
                    payable on or before the first day of each month during the
                    period beginning March 1, 2000 and ending December 31, 2004.

                                       3
<PAGE>

                    All such Base Monthly Rent shall be payable by Lessee in
                    accordance with the terms of Paragraph V hereof.

     IV.  EXPANSION SPACE BASE MONTHLY RENT.
          ---------------------------------

          A.   As of the Expansion Effective Date and in addition to the
               Original Premises Base Monthly Rent, the schedule of monthly
               installments of Base Monthly Rent payable with respect to the
               Expansion Space from and after the Expansion Effective Date is
               the following:

               Lessee shall pay Lessor the sum of One Hundred Fifty-One Thousand
               Eighty-Two and 97/100 Dollars ($151,082.97) as Base Monthly Rent
               for the Expansion Space for the Extended Term in sixty-one (61)
               monthly installments as follows:

               1.   Sixty-One (61) equal installments of $2,476.77 each payable
                    for the period from December 1, 1999 to December 31, 2004;

               Lessor and Lessee acknowledge that the foregoing schedule is
               based on the assumption that the Expansion Effective Date is the
               Target Expansion Effective Date. If the Expansion Effective Date
               is postponed, the beginning date set forth above with respect to
               the payment of any installment(s) of Base Monthly Rent shall be
               appropriately adjusted on a per diem basis and set forth in a
               confirmation letter to be prepared by Lessor. All such Base
               Monthly Rent shall be payable by Lessee in accordance with the
               terms of Paragraph VI hereof.

     V.   ORIGINAL PREMISES ADDITIONAL RENT Effective as of the Extension Date,
          ---------------------------------
          Exhibit A "ADDITIONAL RENT" of the Lease is deleted in its entirety
          and the following shall be in effect for the Original Premises for the
          Extended Term:

          A.   With the lease year commencing January 1, 2000, Lessee shall pay
               to Lessor as additional rent the amount (the "Excess") by which
               the sum of the Lessee's Pro Rata Share of (1) the Taxes
               (hereinafter defined) for the applicable calendar year and (2)
               the Expenses (hereinafter defined) for the applicable calendar
               year exceeds $6.50 per rentable square foot (the "Expense Stop");
               provided Lessee shall not be entitled to a credit of Lessee's Pro
               Rata Share is less than $6.50 per rentable square foot. For
               purposes hereof,

               (i)    "Expenses" shall mean all Basic Costs (as hereinafter
                      defined but excluding Taxes and all janitorial costs
                      except those associated with the common areas of the
                      Building, and

               (ii)   "Lessee's Pro Rata Share" for the Original Premises means
                      2.08%

                                       4
<PAGE>

               As soon as is practical following the end of each calendar year,
               commencing December 31, 2000, Lessor shall furnish to Lessee a
               statement (the "Annual Statement") of Lessor's actual Basic Costs
               and the actual Excess for the previous calendar year. Not later
               than thirty (30) days after Lessee's receipt of the Annual
               Statement, Lessee will pay to Lessor, as additional rent, the
               Excess stated in the Annual Statement.

          B.   Basic Costs shall mean all costs and expenses paid or incurred in
               each calendar year in connection with operating, maintaining,
               repairing, managing and owning the Building and the Property,
               including, but not limited to, the following:

               1.   All labor costs for all persons performing services required
                    or utilized in connection with the operation, repair,
                    replacement and maintenance of and control of access to the
                    Building and the Property, including, but not limited to,
                    amounts incurred for wages, salaries and other compensation
                    for services, payroll, social security, insurance, uniforms,
                    training, disability benefits, pensions, hospitalization,
                    retirement plans, group insurance or any other similar or
                    like expenses or benefits.

               2.   All management fees, the cost of equipping and maintaining a
                    management office at the Building, accounting services,
                    legal fees not attributable to leasing and collection
                    activity, and all other administrative costs relating to the
                    Building and the Property. If management services are not
                    provided by a third party, Lessor shall be entitled to a
                    management fee comparable to that due and payable to third
                    parties provided Lessor or management companies owned by, or
                    management divisions of, Lessor perform actual management
                    services of a comparable nature and type as normally would
                    be performed by third parties.

               3.   All rental and/or purchase costs of materials, supplies,
                    tools and equipment used in the operation, repair,
                    replacement, and maintenance and the control of access to
                    the Building and the Property.

               4.   All amounts charged to Lessor by contractors and/or
                    suppliers for services, replacement parts, components,
                    materials, equipment and supplies furnished in connection
                    with the operation, repair, maintenance, replacement of and
                    control of access to any part of the Building, or Property
                    generally, including the heating, air conditioning,
                    ventilating, plumbing, electrical, elevator and other
                    systems and equipment. At Lessor's option, major repair
                    items may be amortized over a period of up to five (5)
                    years.

               5.   All premiums and deductibles paid by Lessor for fire and
                    extended coverage insurance, earthquake and extended
                    coverage insurance, liability and extended coverage
                    insurance, rental loss insurance, elevator insurance, boiler
                    insurance

                                       5
<PAGE>

                    and other insurance customarily carried from time to time by
                    lessors of comparable office buildings or required to be
                    carried by Lessor's Mortgagee.

               6.   Charges for all utilities, including, but not limited to,
                    water, electricity, gas and sewer, but excluding those
                    charges for which Lessor is otherwise reimbursed by lessees.

               7.   "Taxes," which for purposes hereof, shall mean: (a) all real
                    estate taxes and assessments on the Property, the Building
                    or the Premises, and taxes and assessments levied in
                    substitution of supplementation in whole or in part of such
                    taxes, (b) all personal property taxes for the Building's
                    personal property, including license expense, (c) all taxes
                    imposed on services of Lessor's agents and employees, (d)
                    all costs and fees incurred in connection with seeking
                    reductions in or refunds in Taxes including, without
                    limitations, any costs incurred by Lessor to challenge the
                    tax valuation of the Building, but excluding income taxes.
                    For the purpose of determining real estate taxes and
                    assessments for any given calendar year, the amount to be
                    included in Taxes for such year shall be as follows: (1)
                    with respect to any special assessment that is payable in
                    installments, Taxes for such year shall include the amount
                    of the installment (and any interest) due and payable during
                    such year; and (2) with respect to all other real estate
                    taxes, Taxes for such year shall, at Lessor's election,
                    include either the amount accrued, assessed or otherwise
                    imposed for such year or the amount due and payable for such
                    year, provided that Lessor's election shall be applied
                    consistently throughout the Lease Term. If a reduction in
                    Taxes is obtained for any year of the Lease Term during
                    which Lessee paid its Pro Rata Share of Basic Costs, then
                    Basic Costs for such year will be retroactively adjusted and
                    Lessor shall provide Lessee with a credit, if any, based
                    upon such adjustments. Likewise, if a reduction is
                    subsequently obtained for the tax component of Basic Costs,
                    Basic Costs shall be restated and the Excess for all
                    subsequent years recomputed. Lessee shall pay Lessor
                    Lessee's Pro Rata Share of any such increase in the Excess
                    within thirty (30) days after Lessee's receipt of a
                    statement therefor from Lessor.

               8.   All landscape expenses and costs of maintaining, repairing,
                    resurfacing and striping of the parking area and garages of
                    the Property, if any.

               9.   Cost of all maintenance service agreements, including those
                    for equipment, alarm service, window cleaning, drapery or
                    Venetian blind cleaning, janitorial services, pest control,
                    uniform supply, plant maintenance, landscaping, any parking
                    equipment and rubbish removal and/or disposal fees.

               10.  Cost of all other repairs, replacements and general
                    maintenance of the Property and Building, neither specified
                    above nor directly billed to lessees.

                                       6
<PAGE>

               11.  The amortized cost of capital improvements made to the
                    Building or the Property which are: (a) primarily for the
                    purpose of reducing operating expense costs or otherwise
                    improving the operating efficiency of the Property or
                    Building; or (b) required to comply with any laws, rules or
                    regulations of any governmental authority or a requirement
                    of Lessor's insurance carrier. The cost of such capital
                    improvement shall be amortized over a period of five (5)
                    years and shall, at Lessor's option, include interest at a
                    rate that is reasonably equivalent to the interest rate that
                    Lessor would be required to pay to finance the cost of the
                    capital improvement in question as of the date such capital
                    improvement is performed, provided if the payback period of
                    any capital improvement is less than five (5) years, Lessor
                    may amortize the cost of such capital improvement over the
                    payback period.

               12.  Any other expense or charge of any nature whatsoever which,
                    in accordance with general industry practice with respect to
                    the operation of a first-class office building, would be
                    construed as an operating expense.

                    If the Building is not at least ninety-five percent (95%)
                    occupied during any calendar year of the Lease Term or if
                    Lessor is not supplying services to at least ninety-five
                    percent (95%) of the total Rentable Area of the Building at
                    any time during any calendar year of the Lease Term, actual
                    Basic Costs for purposes hereof shall be determined as if
                    the Building had been ninety-five percent (95%) occupied and
                    Lessor had been supplying services to ninety-five percent
                    (95%) of the Rentable Area of the Building during such year.

          C.   If Basic Costs for any calendar year increase by more than five
               percent (5%) over Basic Costs for the immediately preceding
               calendar year, Lessee, within ninety (90) days after receiving
               the Annual Statement for a particular calendar year, shall have
               the right to provide Lessor with written notice (the "Review
               Notice") of its intent to review Lessor's books and records
               relating to the Basic Costs for such calendar year. Within a
               reasonable time after receipt of a timely Review Notice, Lessor
               shall make such books and records available to Lessee or Lessee's
               agent for its review at either Lessor's home office or the office
               of the Building, provided that if Lessee retains an agent to
               review Lessor's books and records for any calendar year, such
               agent must be CPA firm licensed to do business in the state in
               which the Building is located. If Lessee fails to give Lessor
               written notice of objection within thirty (30) days after its
               review or fails to provide Lessor with a Review Notice within the
               ninety (90) day period provided above, Lessee shall be deemed to
               have approved Lessor's Annual Statement in all respects and shall
               thereafter be barred from raising any claims with respect
               thereto. Any information obtained by Lessee pursuant to the
               provisions of this Section shall be treated as confidential.

                                       7
<PAGE>

     VI.  EXPANSION SPACE ADDITIONAL RENT Effective as of the Expansion
          -------------------------------
          Effective Date, Lessee shall pay to Lessor Additional Rent for the
          Expansion Space for the Extended Term in accordance with the
          following:

          A.   With the lease year commencing January 1, 2000, Lessee shall pay
               to Lessor as additional rent the amount (the "Excess") by which
               the sum of the Lessee's Pro Rata Share of (1) the Taxes
               (hereinafter defined) for the applicable calendar year and (2)
               the Expenses (hereinafter defined) for the applicable calendar
               year exceeds $6.70 per rentable square foot (the "Expense Stop");
               provided Lessee shall not be entitled to a credit of Lessee's Pro
               Rata Share is less than $6.70 per rentable square foot. For
               purposes hereof,

               (i)    "Expenses" shall mean all Basic Costs (as hereinafter
                      defined but excluding Taxes), and

               (ii)   "Lessee's Pro Rata Share" for the Expansion Space means
                      .765%

               As soon as is practical following the end of each calendar year,
               commencing December 31, 2000, Lessor shall furnish to Lessee a
               statement (the "Annual Statement") of Lessor's actual Basic Costs
               and the actual Excess for the previous calendar year. Not later
               than thirty (30) days after Lessee's receipt of the Annual
               Statement, Lessee will pay Lessor, as additional rent, the Excess
               stated in the Annual Statement.

          B.   Basic Costs shall mean all costs and expenses paid or incurred in
               each calendar year in connection with operating, maintaining,
               repairing, managing and owning the Building and the Property,
               including, but not limited to, the following:

               1.   All labor costs for all persons performing services required
                    or utilized in connection with the operation, repair,
                    replacement and maintenance of and control of access to the
                    Building and the Property, including, but not limited to,
                    amounts incurred for wages, salaries and other compensation
                    for services, payroll, social security, insurance, uniforms,
                    training, disability benefits, pensions, hospitalization,
                    retirement plans, group insurance or any other similar or
                    like expenses or benefits.

               2.   All management fees, the cost of equipping and maintaining a
                    management office at the Building, accounting services,
                    legal fees not attributable to leasing and collection
                    activity, and all other administrative costs relating to the
                    Building and the Property. If management services are not
                    provided by a third party, Lessor shall be entitled to a
                    management fee comparable to that due and payable to third
                    parties provided Lessor or management companies owned by, or
                    management division of, Lessor perform actual management

                                       8
<PAGE>

                    services of a comparable nature and type as normally would
                    be performed by third parties.

               3.   All rental and/or purchase costs of materials, supplies,
                    tools and equipment used in the operation, repair,
                    replacement, and maintenance and the control of access to
                    the Building and the Property.

               4.   All amounts charged to Lessor by contractors and/or
                    suppliers for services, replacement parts, components,
                    materials, equipment and supplies furnished in connection
                    with the operation, repair, maintenance, replacement of and
                    control of access to any part of the Building, or Property
                    generally, including the heating, air conditioning,
                    ventilating, plumbing, electrical, elevator and other
                    systems and equipment. At Lessor's option, major repair
                    items may be amortized over a period of up to five (5)
                    years.

               5.   All premiums and deductibles paid by Lessor for fire and
                    extended coverage insurance, earthquake and extended
                    coverage insurance, liability and extended coverage
                    insurance, rental loss insurance, elevator insurance, boiler
                    insurance and other insurance customarily carried from time
                    to time by lessors of comparable office buildings or
                    required to be carried by Lessor's Mortgagee.

               6.   Charges for all utilities, including, but not limited to,
                    water, electricity, gas and sewer, but excluding those
                    charges for which Lessor is otherwise reimbursed by Lessees.

               7.   "Taxes," which for purposes hereof, shall mean: (a) all real
                    estate taxes and assessments on the Property, the Building
                    or the Premises, and taxes and assessments levied in
                    substitution of supplementation in whole or in part of such
                    taxes, (b) all personal property taxes for the Building's
                    personal property, including license expense, (c) all taxes
                    imposed on services of Lessor's agents and employees, (d)
                    all costs and fees incurred in connection with seeking
                    reductions in or refunds in Taxes including, without
                    limitations, any costs incurred by Lessor to challenge the
                    tax valuation of the Building, but excluding income taxes.
                    For the purpose of determining real estate taxes and
                    assessments for any given calendar year, the amount to be
                    included in Taxes for such year shall be as follows: (1)
                    with respect to any special assessment that is payable in
                    installments, Taxes for such year shall include the amount
                    of the installment (and any interest) due and payable during
                    such year; and (2) with respect to all other real estate
                    taxes, Taxes for such year shall, at Lessor's election,
                    include either the amount accrued, assessed or otherwise
                    imposed for such year or the amount due and payable for such
                    year, provided that Lessor's election shall be applied
                    consistently throughout the Lease Term. If a reduction in
                    Taxes is obtained for any year of the Lease Term during
                    which Lessee paid its Pro Rata Share of Basic Costs, then
                    Basic

                                       9
<PAGE>

                    Costs for such year will be retroactively adjusted and
                    Lessor shall provide Lessee with a credit, if any, based
                    upon such adjustments. Likewise, if a reduction is
                    subsequently obtained for the tax component of Basic Costs,
                    Basic Costs shall be restated and the Excess for all
                    subsequent years recomputed. Lessee shall pay Lessor
                    Lessee's Pro Rata Share of any such increase in the Excess
                    within thirty (30) days after Lessee's receipt of a
                    statement therefor from Lessor.

               8.   All landscape expenses and costs of maintaining, repairing,
                    resurfacing and striping of the parking area and garages of
                    the Property, if any.

               9.   Cost of all maintenance service agreements, including those
                    for equipment, alarm service, window cleaning, drapery or
                    Venetian blind cleaning, janitorial services, pest control,
                    uniform supply, plant maintenance, landscaping, any parking
                    equipment and rubbish removal and/or disposal fees.

               10.  Cost of all other repairs, replacements and general
                    maintenance of the Property and Building, neither specified
                    above nor directly billed to lessees.

               11.  The amortized cost of capital improvements made to the
                    Building or the Property which are: (a) primarily for the
                    purpose of reducing operating expense costs or otherwise
                    improving the operating efficiency of the Property or
                    Building; or (b) required to comply with any laws, rules or
                    regulations of any governmental authority or a requirement
                    of Lessor's insurance carrier. The cost of such capital
                    improvement shall be amortized over a period of five (5)
                    years and shall, at Lessor's option, include interest at a
                    rate that is reasonably equivalent to the interest rate that
                    Lessor would be required to pay to finance the cost of the
                    capital improvement in question as of the date such capital
                    improvement is performed, provided if the payback period of
                    any capital improvement is less than five (5) years, Lessor
                    may amortize the cost of such capital improvement over the
                    payback period.

               12.  Any other expense or charge of any nature whatsoever which,
                    in accordance with general industry practice with respect to
                    the operation of a first-class office building, would be
                    construed as an operating expense.

                    If the Building is not at least ninety-five percent (95%)
                    occupied during any calendar year of the Lease Term or if
                    Lessor is not supplying services to at least ninety-five
                    percent (95%) of the total Rentable Area of the Building at
                    any time during any calendar year of the Lease Term, actual
                    Basic Costs for purposes hereof shall be determined as if
                    the Building had been ninety-five percent (95%) occupied and
                    Lessor had been supplying services to ninety-five percent
                    (95%) of the Rentable Area of the Building during such year.

                                       10
<PAGE>

          C.   If Basic Costs for any calendar year increase by more than five
               percent (5%) over Basic Costs for the immediately preceding
               calendar year, Lessee, within ninety (90) days after receiving
               the Annual Statement for a particular calendar year, shall have
               the right to provide Lessor with written notice (the "Review
               Notice") of its intent to review Lessor's books and records
               relating to the Basic Costs for such calendar year. Within a
               reasonable time after receipt of a timely Review Notice, Lessor
               shall make such books and records available to Lessee or Lessee's
               agent for its review at either Lessor's home office or the office
               of the Building, provided that if Lessee retains an agent to
               review Lessor's books and records for any calendar year, such
               agent must be CPA firm licensed to do business in the state in
               which the Building is located. If Lessee fails to give Lessor
               written notice of objection within thirty (30) days after its
               review or fails to provide Lessor with a Review Notice within the
               ninety (90) day period provided above, Lessee shall be deemed to
               have approved Lessor's Annual Statement in all respects and shall
               thereafter be barred from raising any claims with respect
               thereto. Any information obtained by Lessee pursuant to the
               provisions of this Section shall be treated as confidential.


     VII. IMPROVEMENTS TO EXPANSION SPACE.
          -------------------------------

          A.   Condition of Expansion Space. Lessee has inspected the Expansion
               Space and agrees to accept the same "as is" without any
               agreements, representations, understandings or obligations on the
               part of the Lessor to perform any alterations, repairs or
               improvements, except as may be expressly provided otherwise in
               this Amendment.

          B.   Cost of Improvements to Expansion Space. Provided Lessee is not
               in default, Lessee shall be entitled to receive an improvement
               allowance (the "Expansion Improvement Allowance") in an amount
               not to exceed Twelve Thousand Nine Hundred Sixteen and no/100
               Dollars ($12,916.00) (i.e., $7.06 per rentable square foot of the
               Expansion Space) to be applied toward the cost of performing
               initial construction, alteration or improvement of the Expansion
               Space, including but not limited to the cost of space planning,
               design and related architectural and engineering services. In the
               event the total cost of the initial improvements to the Expansion
               Space exceeds the Expansion Improvement Allowance, Lessee shall
               pay for such excess upon demand. The entire unused balance of the
               Expansion Improvement Allowance, if any, shall accrue to the sole
               benefit of Lessor. Lessor shall pay such Expansion Improvement
               Allowance directly to the contractors retained to perform the
               construction, design or related improvement work to the Expansion
               Space.

          C.   Responsibility for Improvements to Expansion Space. Lessor shall
               enter into a direct contract for the initial improvements to the
               Expansion Space with a general contractor selected by Lessor.
               Lessee shall devote such time in consultation with Lessor or
               Lessor's architect as may be required to provide all information
               Lessor

                                       11
<PAGE>

               deems necessary in order to enable Lessor to complete, and obtain
               Lessee's written approval of, the plans for the initial
               improvements to the Expansion Space in a timely manner. All plans
               for the initial improvements to the Expansion Space shall be
               subject to Lessor's consent, which consent shall not be
               unreasonably withheld. If the cost of such improvements exceeds
               the Expansion Improvement Allowance, then prior to commencing any
               construction of improvements to the Expansion Space, Lessor shall
               submit to Lessee a written estimate setting forth the anticipated
               cost, including but not limited to the cost of space planning,
               design and related architectural and engineering services, labor
               and materials, contractor's fees, and permit fees. Within a
               reasonable time thereafter, Lessee shall either notify Lessor in
               writing of its approval of the cost estimate or specify its
               objections thereto and any desired changes to the proposed
               improvements. In the event Lessee notifies Lessor of such
               objections and desired changes, Lessee shall work with Lessor to
               reach a mutually acceptable alternative cost estimate.

    VIII. EARLY ACCESS TO EXPANSION SPACE. During any period that Lessee shall
          -------------------------------
          be permitted to enter the Expansion Space prior to the Expansion
          Effective Date (e.g., to perform alterations or improvements, if any),
          Lessee shall comply with all terms and provisions of the Lease, except
          those provisions requiring payment of Base Rental or Additional Base
          Rental as to the Expansion Space. If Lessee takes possession of the
          Expansion Space prior to the Expansion Effective Date for any reason
          whatsoever (other than the performance of the work in the Expansion
          Space with Lessor's prior approval), such possession shall be subject
          to all the terms and conditions of the Lease and this Amendment, and
          Lessee shall pay Base Rental and Additional Base Rental as applicable
          to the Expansion Space to Lessor on a per diem basis for each day of
          occupancy prior to the Expansion Effective Date.

     IX.  OTHER PERTINENT PROVISIONS. Lessor and Lessee agree that, effective as
          --------------------------
          of the date hereof (unless different effective date(s) is/are
          specifically referenced in this Section), the Lease shall be amended
          in the following additional respects:

          A.   ARTICLE 29, "Notices" of the Lease shall be modified and amended
               by deleting the section containing the address for Lessor and
               substituting the following therefor:

               "Lessor

               EOP-Summit Limited Partnership
               c/o Equity Office Properties Trust
               1300 Summit Avenue
               Suite 220
               Fort Worth, Texas 76102

               With a copy to Lessor:

                                       12
<PAGE>

               c/o Equity Office Properties Trust
               Two North Riverside Plaza
               Suite 2200
               Chicago, Illinois 60606
               Attention: Regional Counsel - Southwest Region

               Payment of Rent only shall be made payable to:
               Equity Office Properties

               at the following address:

               Equity Office Properties, L.L.C.
               d/b/a Summit Office Park
               P.O. Box 844552
               Dallas, Texas 75284-4552"

          B.   Effective as of the Expansion Effective Date and subject to the
               terms and conditions of ARTICLE C.l.E. of the Second Amendment,
               Lessee agrees to lease from Lessor and Lessor agrees to lease to
               Lessee a total of two (2) additional covered, reserved Spaces and
               four (4) unreserved surface Spaces (the "Additional Spaces") in
               the Parking Area.

          C.   Effective as of the Expansion Effective Date, ARTICLE C.ll. of
               the Second Amendment "Suite 634 Rights" shall be deleted in its
               entirety, and shall be of no further force and effect.

     X.   MISCELLANEOUS.
          --------------

          A.   This Amendment sets forth the entire agreement between the
               parties with respect to the matters set forth herein. There have
               been no additional oral or written representations or agreements.
               Under no circumstances shall Lessee be entitled to any Rent
               abatement, improvement allowance, leasehold improvements, or
               other work to the Premises, or any similar economic incentives
               that may have been provided Lessee in connection with entering
               into the Lease, unless specifically set forth in this Amendment.

          B.   Except as herein modified or amended, the provisions, conditions
               and terms of the Lease shall remain unchanged and in full force
               and effect.

          C.   In the case of any inconsistency between the provisions of the
               Lease and this Amendment, the provisions of this Amendment shall
               govern and control.

          D.   Submission of this Amendment by Lessor is not an offer to enter
               into this Amendment but rather is a solicitation for such an
               offer by Lessee. Lessor shall not

                                       13
<PAGE>

               be bound by this Amendment until Lessor has executed and
               delivered the same to Lessee.

          E.   The capitalized terms used in this Amendment shall have the same
               definitions as set forth in the Lease to the extent that such
               capitalized terms are defined therein and not redefined in this
               Amendment.

          F.   Lessee hereby represents to Lessor that Lessee has dealt with no
               broker other than Kelly Geren & Searcy in connection with this
               Amendment. Lessee agrees to indemnify and hold Lessor, its
               members, principals, beneficiaries, partners, officers,
               directors, employees, mortgagee(s) and agents, and the respective
               principals and members of any such agents (collectively, the
               "Lessor Related Parties") harmless from all claims of any brokers
               other than Kelly Geren & Searcy claiming to have represented
               Lessee in connection with this Amendment. Lessor hereby
               represents to Lessee that Lessor has dealt with no broker other
               than the Richard D. Minker Co. in connection with this Amendment.
               Lessor agrees to indemnify and hold Lessee, its members,
               principals, beneficiaries, partners, officers, directors,
               employees, and agents, and the respective principals and members
               of any such agents (collectively, the "Lessee Related
               Parties")harmless from all claims of any brokers other than the
               Richard D. Minker Co. claiming to have represented Lessor in
               connection with this Third Amendment.

                                       14
<PAGE>

     IN WITNESS WHEREOF, Lessor and Lessee have duly executed this Amendment as
of the day and year first above written.

WITNESSES; ATTESTATION LESSOR:     EOP-SUMMIT LIMITED PARTNERSHIP, a
                                   Delaware limited partnership

                                   By:  EOP-Summit, L.L.C., a Delaware limited
                                        liability company, its general partner

                                        By:  EOP Operating Limited Partnership,
                                             a Delaware limited partnership, its
                                             sole member

                                             By:  Equity Office Properties
                                                  Trust, a Maryland real estate
                                                  investment trust, its managing
                                                  general partner


                                                  By:    /s/ Brad Fricks
                                                         -----------------------
                                                  Name:  Brad Fricks
                                                         -----------------------
                                                  Title: Vice-President- Leasing
                                                         -----------------------


                       LESSEE:     SUMMIT BANCSHARES, INC.


                                   By:    /s/ Bob G. Scott
                                          --------------------------------------
                                   Name:  Bob G. Scott
                                          --------------------------------------
                                   Title: Executive Vice President
                                          --------------------------------------

                                       15
<PAGE>

                                   EXHIBIT A

                                EXPANSION SPACE


     This Exhibit is attached to and made a part of the lease dated as of the
22nd October, 1999, by and between EOP SUMMIT LIMITED PARTNERSHIP, a Delaware
limited partnership ("LESSOR") and SUMMIT BANCSHARES, INC. ("LESSEE") for space
in the Building located at 1300 Summit Avenue, Suite 634, Fort Worth, Texas.


                            1300 SUMMIT OFFICE PARK
                    Tenant Name:    Summit Bancshares, Inc.
                    Suite Number:   634
                    Square Feet:    1,829 rentable

     IN WITNESS WHEREOF, LESSOR AND LESSEE have entered into this Lease as of
the date first written above.

LESSOR:  EOP-SUMMIT LIMITED PARTNERSHIP, a Delaware limited partnership

         By:   EOP-Summit, L.L.C., a Delaware limited liability company, its
               general partner

               By:  EOP Operating Limited Partnership, a Delaware limited
                    partnership, its sole member

                    By:  Equity Office Properties Trust, a Maryland real estate
                         investment trust, its managing general partner

                         By:    /s/ Brad Fricks
                                ------------------------
                         Name:  Brad Fricks
                                ------------------------
                         Title: Vice-President- Leasing
                                ------------------------

LESSEE:  SUMMIT BANCSHARES, INC.

         By:    /s/ Bob G. Scott
                -------------------------
         Name:  Bob G. Scott
                -------------------------
         Title: Executive Vice President
                -------------------------

                                       16