The glass transition temperature (T-g) of a thermosetting polymer such as a structural adhesive depends on the thermal history (cure cycle) as well as the current temperature. The manner in which the T-g of the adhesive changes with different cure cycles and thermal histories was studied in the present paper, after a damping at resonance method of measuring the T-g of the adhesive had been developed previously by the authors. It is shown here that T-g is affected by both the post-cure temperature and its duration, but is more dependent on the temperature. However, prolonged post-curing can cause a decrease in T-g, and thus the appropriate temperature and duration need to be used.