EXHIBIT 4.20

                                    FORM OF
                    AGREEMENT AS TO EXPENSES AND LIABILITIES

         AGREEMENT  dated as of June 21, 2001 between SENIOR HOUSING  PROPERTIES
TRUST,  a Maryland real estate  investment  trust  ("Senior  Housing"),  and SNH
CAPITAL TRUST I, a Maryland business trust (the "Trust").

         WHEREAS,  the Trust intends to issue its 10.125% Common Securities (the
"Common  Securities")  to, and purchase 10.125% Junior  Subordinated  Debentures
(the "Debentures") from, Senior Housing, and to issue and sell its 10.125% Trust
Preferred  Securities (the "Trust Preferred  Securities",  and together with the
Common   Securities,   the   "Trust   Securities")   to  the   purchasers   (the
"Underwriters")  named in Schedule A to the Underwriting  Agreement,  dated June
18,  2001  (the  "Underwriting  Agreement"),  among the  Representatives  of the
several  Underwriters,  the Trust, SNH Capital Trust Holdings and Senior Housing
with such powers,  preferences  and special rights and  restrictions  as are set
forth in the Amended and Restated Trust Agreement of the Trust among SNH Capital
Trust Holdings, State Street Bank and Trust Company ("Property Trustee") and the
Regular  Trustees named therein relating to the Trust dated as of June 21, 2001,
as the same may be amended from time to time (the "Trust Agreement");

         WHEREAS,  Senior  Housing will  directly or  indirectly  own all of the
Common Securities of the Trust and will issue the Debentures;

         NOW, THEREFORE,  in consideration of the purchase by each holder of the
Trust  Preferred  Securities,  which purchase Senior Housing hereby agrees shall
benefit Senior Housing and which purchase  Senior Housing  acknowledges  will be
made in reliance  upon the  execution  and  delivery of this  Agreement,  Senior
Housing and the Trust hereby agree as follows:

                                   ARTICLE I
                                    EXPENSES

         1.1 Payment of Expenses.  In  connection  with the  offering,  sale and
issuance of the  Debentures to the Property  Trustee and in connection  with the
offering,  sale and  issuance  of the  Trust  Securities  by the  Trust,  Senior
Housing, shall:

         (a) pay all costs  and  expenses  relating  to the  offering,  sale and
issuance of the Debentures and the Trust  Securities,  including  commissions to
the Underwriters payable pursuant to the Underwriting Agreement and compensation
of the Trustee under the Indenture pertaining to the Debentures;

         (b) pay all costs and expenses of the Trust (including, but not limited
to, costs and expenses  relating to the  organization of the Trust, the fees and
expenses  of the  Property  Trustee,  the costs  and  expenses  relating  to the
operation  of the Trust,  including  without  limitation,  costs and expenses of
accountants,  attorneys,  statistical  or  bookkeeping  services,  expenses  for
printing and engraving and computing or accounting  equipment,  paying agent(s),
registrar(s),  transfer  agent(s),  duplicating,  travel and telephone and other
telecommunications  expenses and costs and expenses  incurred in connection with
the acquisition, financing, and disposition of Trust assets);
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         (c) pay all  costs  and  expenses  related  to the  enforcement  by the
Property Trustee of the rights of the registered  holders of the Trust Preferred
Securities;

         (d) be primarily  liable for any  indemnification  obligations  arising
with respect to the Trust Agreement and
the Underwriting Agreement; and

         (e) pay any and all taxes and all liabilities,  costs and expenses with
respect to such taxes of the Trust (but not including  withholding taxes imposed
on holders of Trust Preferred Securities or Common Securities of the Trust).

                                   ARTICLE II
                           GUARANTEE BY SENIOR HOUSING

         2.1 Guarantee by Senior  Housing.  Subject to the terms and  conditions
hereof, Senior Housing hereby irrevocably and unconditionally guarantees to each
person  or  entity to whom the Trust is now or  hereafter  becomes  indebted  or
liable (the  "Beneficiaries") the full payment,  when and as due, of any and all
Obligations  (as  hereinafter  defined) to such  Beneficiaries.  As used herein,
"Obligations" means any costs,  expenses or liabilities of the Trust, other than
obligations of the Trust to pay to holders of any Trust Preferred Securities the
amounts  due  such  holders  pursuant  to  the  terms  of  the  Trust  Preferred
Securities.  This  Agreement  is  intended  to be for the  benefit of, and to be
enforceable by, all such  Beneficiaries,  whether or not such Beneficiaries have
received notice hereof.

         2.2 Terms of Agreement.  This  Agreement  shall  terminate and be of no
further  force and effect  upon the later of (a) the date on which full  payment
has been made of all amounts  payable to all holders of all the Trust  Preferred
Securities (whether upon redemption, liquidation, exchange or otherwise) and (b)
the date on which there are no Beneficiaries remaining;  provided, however, that
this Agreement  shall  continue to be effective or shall be  reinstated,  as the
case may be, if at any time any  holder  of Trust  Preferred  Securities  or any
Beneficiary  must  restore  payment of any sums paid  under the Trust  Preferred
Securities, under any Obligation, under the Trust Preferred Securities Guarantee
Agreement  dated the date  hereof by Senior  Housing  and State  Street Bank and
Trust  Company,  as guarantee  trustee,  or under this  Agreement for any reason
whatsoever.  This  Agreement  is  continuing,  irrevocable,   unconditional  and
absolute.

         2.3 Waiver of Notice. Senior Housing hereby waives notice of acceptance
of this  Agreement and of any  Obligation to which it applies or may apply,  and
Senior Housing hereby waives presentment, demand for payment, protest, notice of
nonpayment,  notice of dishonor,  notice of redemption and all other notices and
demands.

         2.4 No Impairment. The obligations, covenants, agreements and duties of
Senior Housing under this  Agreement  shall in no way be affected or impaired by
reason of the happening from time to time of any of the following:

         (a) the  extension  of time for the  payment by the Trust of all or any
portion of the Obligations or for the performance of any other obligation under,
arising out of, or in connection with, the Obligations;

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         (b) any  failure,  omission,  delay or lack of diligence on the part of
the Beneficiaries to enforce, assert or exercise any right, privilege,  power or
remedy  conferred on the  Beneficiaries  with respect to the  Obligations or any
action on the part of the Trust granting indulgence or extension of any kind; or

         (c) the voluntary or involuntary liquidation,  dissolution, sale of any
collateral, receivership,  insolvency, bankruptcy, assignment for the benefit of
creditors, reorganization,  arrangement, composition or readjustment of debt of,
or other similar  proceedings  affecting,  the Trust or any of the assets of the
Trust.

         There shall be no obligation of the Beneficiaries to give notice to, or
obtain the consent of,  Senior  Housing with respect to the  happening of any of
the foregoing.

         2.5  Enforcement.  A Beneficiary  may enforce this  Agreement  directly
against Senior Housing, and Senior Housing waives any right or remedy to require
that any  action be  brought  against  the  Trust or any other  person or entity
before proceeding against Senior Housing.

         2.6  Subrogation.  Senior Housing shall be subrogated to all rights (if
any) of the Trust in respect of any amounts paid to the  Beneficiaries by Senior
Housing under this Agreement;  provided,  however, that Senior Housing shall not
(except to the extent  required by mandatory  provisions  of law) be entitled to
enforce or exercise any rights which it may acquire by way of subrogation or any
indemnity, reimbursement or other agreement, in all cases as a result of payment
under this Agreement,  if, at the time of any such payment,  any amounts are due
and unpaid under this Agreement.

                                  ARTICLE III
                                 BINDING EFFECT

         3.1 Binding  Effect.  All guarantees  and agreements  contained in this
Agreement  shall  bind  the  successors,   assigns,   receivers,   trustees  and
representatives  of  Senior  Housing  and  shall  inure  to the  benefit  of the
Beneficiaries.

         3.2  Amendment.  So long as there remains any  Beneficiary or any Trust
Preferred Securities of any series are outstanding,  this Agreement shall not be
modified or amended in any manner adverse to such  Beneficiary or to the holders
of the Trust Preferred Securities.

         3.3 Notices.  Any notice,  request or other  communication  required or
permitted to be given hereunder shall be given in writing by delivering the same
by  personal  delivery,  by  facsimile  transmission  or  by  first-class  mail,
addressed as follows (and if so given,  shall be deemed given when so delivered,
upon receipt of confirmation  if by facsimile,  or three days after mailed if by
first-class mail):

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         If to the Trust to:

                  SNH Capital Trust I
                  400 Centre Street
                  Newton, MA  02458
                  Attention:  Barry M. Portnoy
                  Telecopy No.:  (617) 969-1437

         with a copy to:

                  Senior Housing Properties Trust
                  400 Centre Street
                  Newton, MA  02458
                  Attention:  David J. Hegarty
                  Telecopy No.:  (617) 796-8349

         3.4 Governing  Law. This  Agreement  shall be governed by and construed
and  interpreted in accordance  with the laws of the State of Maryland  (without
regard to conflict of laws principles).


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         THIS AGREEMENT is executed as of the day and year first above written.

                              SENIOR HOUSING PROPERTIES TRUST


                              By:
                                  -----------------------------------------
                                   David J. Hegarty
                                   President



                              SNH CAPITAL TRUST I


                              By:
                                 ------------------------------------------
                                  Barry M. Portnoy
                                  Trustee


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