EXHIBIT 5

                              SULLIVAN & WORCESTER LLP
                               ONE POST OFFICE SQUARE
                             BOSTON, MASSACHUSETTS 02109
                                   (617) 338-2800
                                FAX NO. 617-338-2880
     IN WASHINGTON, D.C.                                   IN NEW YORK CITY
1025 CONNECTICUT AVENUE, N.W.                              767 THIRD AVENUE
   WASHINGTON, D.C. 20036                              NEW YORK, NEW YORK 10017
       (202) 775-8190                                       (212) 486-8200
    FAX NO. 202-293-2275                                 FAX NO. 212-758-2151






                                                  May 8, 1998



SIS Bancorp, Inc.
1441 Main Street
Springfield, Massachusetts 01102

         Re:      Registration Statement on Form S-8 of 340,000 shares of Common
                  Stock, par value $0.01 per share

Ladies and Gentlemen:

         In connection with the  registration  under the Securities Act of 1933,
as amended (the "Act"), by SIS Bancorp,  Inc., a Massachusetts  corporation (the
"Company"), of 340,000 shares (the "Registered Shares") of its Common Stock, par
value $.01 per share ("Common Stock"),  all of which Registered Shares are to be
offered by the Company,  the  following  opinion is furnished to you to be filed
with the Securities and Exchange  Commission (the  "Commission") as Exhibit 5 to
the Company's registration statement on Form S-8 (the "Registration  Statement")
under  the  Act.  The  Registered  Shares  are to be  offered  on a  delayed  or
continuous  basis pursuant to Rule 415 under the Act in connection  with options
granted  under the  Company's  Stock Option Plan,  as amended and restated  (the
"Plan").

         We assume that the number and  issuance  of options to be granted  from
time to time  pursuant  to the Plan  have been or will be  authorized  by proper
action of the Company's Board of Directors or the proper  committee  thereof and
that the number,  issuance and sale of the Registered  Shares to be offered from
time to time  pursuant to the  exercise of such options  will be  determined  in
accordance  with the  parameters  described in the Plan, in accordance  with the
Company's Articles of Organization,  as amended (the "Articles"), and applicable
Massachusetts  law.  We  further  assume  that  prior  to  the  issuance  of any
Registered Shares, there will exist, under the Articles, the requisite number of
authorized  shares of Common Stock for such issuance  which are unissued and are
not otherwise reserved for issuance.

         We have  acted  as  counsel  to the  Company  in  connection  with  the
Registration Statement,  and we have examined originals or copies,  certified or
otherwise  identified to our satisfaction,  of the Registration  Statement,  the
Articles as presently in effect, corporate records,  certificates and statements
of officers and  accountants  of the Company and of public  officials,  and such
other documents as we have considered  necessary in order to furnish the opinion
hereinafter set forth.





SIS Bancorp, Inc.
May 8, 1998
Page 2

         This   opinion  is  limited  to  the  laws  of  the   Commonwealth   of
Massachusetts  and we express no  opinion  with  respect to the law of any other
jurisdiction.

         Based on and subject to the foregoing, we are of the opinion that, upon
the  issuance by the Company of  Registered  Shares  pursuant to the exercise of
options  granted under the Plan and upon delivery of  certificates  representing
such  Registered  Shares  in  the  manner  contemplated  by  the  Plan  and  the
authorization relating thereto by the Company's Board of Directors or the proper
committee  thereof,  the Registered Shares represented by such certificates will
be duly authorized, validly issued, fully paid and nonassessable by the Company.

         We hereby  consent to the  filing of this  opinion as an exhibit to the
Registration  Statement  and to the  reference  to our  firm  in the  Prospectus
forming a part of the Registration  Statement. In giving such consent, we do not
thereby  admit that we come  within the  category  of persons  whose  consent is
required  under  Section  7 of the  Act  or the  rules  and  regulations  of the
Commission promulgated thereunder.

                                         Very truly yours,

                                         /s/ Sullivan & Worcester LLP

                                         SULLIVAN & WORCESTER LLP