<Page>

                                                                   EX-99.906CERT

                                  CERTIFICATION

                             Pursuant to Section 906
                                     of the
                           Sarbanes-Oxley Act of 2002


Name of Registrant:   ING SENIOR INCOME FUND

Date of Form N-CSR:   PERIOD ENDED AUGUST 31, 2003 TO BE FILED NOVEMBER 6, 2003


     The undersigned, the principal executive officer of the above named
registrant (the "Fund"), hereby certifies that, with respect to the Form N-CSR
referred to above, to the best of his knowledge and belief, after reasonable
inquiry:

     1.   such Form N-CSR fully complies with the requirements of section 13(a)
or 15(d) of the Securities Exchange Act of 1934; and

     2.   the information contained in such Form N-CSR fairly presents, in all
material respects, the financial condition and results of operations of the
Fund.

A signed original of this written statement required by Section 906 has been
provided to ING Senior Income Fund and will be retained by ING Senior Income
Fund and furnished to the Securities and Exchange Commission or its staff upon
request.

     IN WITNESS WHEREOF, the undersigned has executed this Certification below,
as of this 6th day of November, 2003.


                                       /s/ James M. Hennessy
                                       --------------------------------
                                              James M. Hennessy

<Page>

                                                                   EX-99.906CERT

                                  CERTIFICATION

                             Pursuant to Section 906
                                     of the
                           Sarbanes-Oxley Act of 2002


Name of Registrant:   ING SENIOR INCOME FUND

Date of Form N-CSR:   PERIOD ENDED AUGUST 31, 2003 TO BE FILED NOVEMBER 6, 2003


     The undersigned, the principal financial officer of the above named
registrant (the "Fund"), hereby certifies that, with respect to the Form N-CSR
referred to above, to the best of his knowledge and belief, after reasonable
inquiry:

     1.   such Form N-CSR fully complies with the requirements of section 13(a)
or 15(d) of the Securities Exchange Act of 1934; and

     2.   the information contained in such Form N-CSR fairly presents, in all
material respects, the financial condition and results of operations of the
Fund.

A signed original of this written statement required by Section 906 has been
provided to ING Senior Income Fund and will be retained by ING Senior Income
Fund and furnished to the Securities and Exchange Commission or its staff upon
request.

     IN WITNESS WHEREOF, the undersigned has executed this Certification below,
as of this 6th day of November, 2003.


                                       /s/ Michael J. Roland
                                       --------------------------------
                                              Michael J. Roland