BAKER & McKENZIE
                                Attorneys at Law

                              One Prudential Plaza
                             130 East Randolph Drive
                             Chicago, Illinois 60601


                                February 26, 1999

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-8 (the  "Registration  Statement") under the
Securities Act of 1933, as amended,  of an aggregate of 1,250,000  shares of the
Company's  Class A Common  Stock,  $.005 par value per share (the "Stock") to be
issued by the  Company  pursuant  to the  Company's  1999 Stock  Option Plan and
1999 Employee  Stock  Purchase Plan  (together,  the "Plans").  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 laws as
we have deemed necessary to the rendering of the opinions expressed below.

         Based upon the  foregoing,  we are of the  opinion  that the  1,250,000
shares of Stock to be issued by the Company,  as  described  in the Plans,  will
be legally  issued,  fully paid and  non-assessable  when issued and paid for in
the manner contemplated in the Plans.

         We hereby  consent to the  filing of this  opinion as an exhibit to the
Registration Statement.


                                                              Very truly yours,



                                                              Baker & McKenzie