

                                BAKER & McKENZIE
                                Attorneys at Law

                              One Prudential Plaza
                            130 East Randolph Drive
                            Chicago, Illinois 60601

                                 June 8, 1998

Board of Directors
Transaction Systems Architects, Inc.
224 South 108th Avenue
Omaha, Nebraska  68154

     Re:   Transaction Systems Architects, Inc. (the "Company")

Gentlemen:

     We have acted as your counsel in  connection  with the  registration,  on a
Registration  Statement  on Form S-4 (the  "Registration  Statement")  under the
Securities Act of 1933, as amended, of 2,500,000 shares of the Company's Class A
Common  Stock,  $.005 par value per  share  (the  "Stock"),  to be issued by the
Company  from  time to time in  connection  with  acquisitions  by the  Company,
directly or indirectly, of other businesses or properties, or interests therein.
We have  reviewed  the  Registration  Statement,  the charter and by-laws of the
Company,  corporate  proceedings  of the  Board  of  Directors  relating  to the
issuance of the shares of Stock, and such other documents, corporate records and
questions of law as we have deemed  necessary  to the  rendering of the opinions
expressed below.

     Based upon the foregoing,  we are of the opinion that the 2,500,000  shares
of Stock to be issued by the Company as described in the Registration  Statement
are duly authorized and , when issued and paid for in the manner contemplated in
the  Registration  Statement  and upon approval by the Board of Directors of the
acquistion  in which  shares of Stock  will be issued,  will be legally  issued,
fully paid and non-assessable shares of Class A Common Stock of the Company.

     We hereby  consent to the  reference  to our firm under the caption  "Legal
Matters" in the  prospectus  included in the  Registration  Statement and to the
filing of this opinion as an exhibit thereto.

                                                       Very truly yours,

                                                       BAKER & McKENZIE



