The reaction of SO2 with fly ash in the presence of O-2 and H2O involves a series of reactions that lead to the formation of SO3 and eventually H2SO4. Homogeneous experiments were conducted to evaluate the effects of the procedural variables, i.e., temperature, gas concentrations, and residence time, on the post-combustion conversion of SO2 to SO3. The results were compared to existing global kinetics and found to be dependent upon SO2, O-2, residence time, and temperature and independent of H2O content. For a residence time of 1 s, temperatures of about 900 degrees C are needed to have an observable conversion of SO2 to SO3. Literature suggested that the conversion of SO2 to SO3 is dependent upon the iron oxide content of the fly ash. Experiments using three different fly ash samples from Australian sub-bituminous coals were used to investigate the catalytic effects of fly ash on SO2 conversion to SO3 at a temperature range of 4001000 degrees C. It was observed that fly ash acts as a catalyst in the formation of SO3, with the largest conversion occurring at 700 degrees C. A homogeneous reaction at 700 degrees C, without fly ash present, converted 0.10% of the available SO2 to SO3. When fly ash was present, the conversion increased to 1.78%. The catalytic effect accounts for roughly 95% of the total conversion. Average SO3/SO2 conversion values between fly ash derived from air and oxy-fuel firing and under different flue gas environments were found to be similar.