Three types of platinum catalysts, Pt/SiO2, Pt/γ-Al2O3 and Pt/ZrO2 were examined for the oxidation of NO to NO2 of SO2 to SO3. The activity order for both oxidation reactions was found to be: Pt/SiO2>Pt/γ-Al2O3>Pt/ZrO2. The effect of Pt loading and Pt dispersion on the catalytic activity and selectivity was examined. Over Pt/SiO2, the specific activities were found to be strongly size-dependent, the larger Pt particles exhibiting higher specific activity than the smaller ones. Over the Pt/γ-Al2O3 catalysts, however, the size-dependence appeared to be much less significant than that of the Pt/SiO2 catalysts. The results for Pt/ZrO2 catalysts, however, showed that the specific activities for both reactions were most probably size-independent. The catalytic selectivity for the oxidation of NO relative to that of SO2 appeared to be size-independent for all the catalysts. The interaction of NO, NO2 and SO2 with the catalysts and the effect of the support on the interaction were investigated using TPD technique.