NO oxidation was studied over Pt/CeO2 and Pt/SiO2 catalysts. Apparent activation energies (E-a) of 31.4 and 40.6 kJ/mole were determined for Pt/CeO2 and Pt/SiO2, respectively, while reaction orders for NO and O-2 were fractional and positive for both catalysts. Pre-treatment of the catalysts with SO2 caused a decrease in the E-a values, while the reaction orders were only slightly changed. In situ DRIFTS measurements indicated that high concentrations of nitrate species were formed on the surface of Pt/CeO2 during NO oxidation, while almost no surface species could be detected on Pt/SiO2. The addition of SO2 resulted in the formation of a highly stable sulfate at the expense of nitrate species and caused an irreversible loss of catalytic activity for Pt/CeO2.