Exhibit 31.2
                         CERTIFICATIONS

I, Patrick W. Keene, certify that:

1.  I  have  reviewed this quarterly report on Form 10-Q  of  AEI
Income & Growth Fund XXI Limited Partnership;

2. Based on my knowledge, this report does not contain any untrue
statement  of  a material fact or omit to state a  material  fact
necessary  to  make  the  statements  made,  in  light   of   the
circumstances  under  which  such  statements  were   made,   not
misleading with respect to the period covered by this report;

3.  Based  on my knowledge; the financial statements,  and  other
financial information included in this report, fairly present  in
all  material  respects  the  financial  condition,  results   of
operations and cash flows of the registrant as of, and  for,  the
periods presented in this report;

4.   The  registrant's  other  certifying  officers  and  I   are
responsible for establishing and maintaining disclosure  controls
and  procedures (as defined in Exchange Act Rules  13a-15(e)  and
15d-15(e)) for the registrant and have;

       a)   designed such disclosure controls and procedures,  or
caused  such  disclosure controls and procedures to  be  designed
under  our  supervision,  to  ensure  that  material  information
relating   to   the   registrant,  including   its   consolidated
subsidiaries,  is  made  known  to  us  by  others  within  those
entities, particularly during the period in which this report  is
being prepared;

        b)   evaluated  the  effectiveness  of  the  registrant's
disclosure  controls and procedures and presented in this  report
our   conclusions  about  the  effectiveness  of  the  disclosure
controls  and procedures, as of the end of the period covered  by
this report based on such evaluation; and

      c)  disclosed in this report any change in the registrant's
internal  control over financial reporting that  occurred  during
the  registrant's  most recent fiscal quarter  (the  registrant's
fourth  fiscal quarter in the case of an annual report) that  has
materially  affected,  or  is  reasonably  likely  to  materially
affect,   the   registrant's  internal  control  over   financial
reporting;

5.   The  registrant's  other  certifying  officers  and  I  have
disclosed,  based  on  our  most recent evaluation,  of  internal
control  over  financial reporting, to the registrant's  auditors
and  the  audit committee of registrant's board of directors  (or
persons performing the equivalent functions):

      a)  all significant deficiencies and material weaknesses in
the  design  or  operation  of internal controls  over  financial
reporting  which  are reasonably likely to adversely  affect  the
registrant's  ability  to record, process, summarize  and  report
financial data and have identified for the registrant's  auditors
any  material  weaknesses  in  internal  control  over  financial
reporting; and

       b)   any  fraud,  whether or not material,  that  involves
management or other employees who have a significant role in  the
registrant's internal controls.




Dated: August 8, 2008     /s/ Patrick W Keene
                              Patrick W. Keene, Chief Financial Officer
                              AEI Fund Management XXI, Inc.
                              Managing General Partner