The catalytic activities of Pt and Au supported on TiO2 were compared with respect to the oxidation of CO and propane. While the Au catalysts showed higher activities for CO oxidation, the Pt catalysts were more active for propane combustion. A strong de-activation of the CO oxidation activity by SO2 was observed only over the TiO2 -supported Au catalyst, indicating that SO2 can block the active sites for CO oxidation over Au catalysts. The results are consistent with a model in which the perimeter sites have a special role in the CO oxidation reaction over Au catalysts. (C) 2000 Elsevier Science B.V. All rights reserved.