The effect of the presence of sulfur on the state of the catalytic surface during oxidation reactions on supported Pt catalysts has been investigated by kinetic studies and operando infrared (IR) and in situ extended X-ray absorption fine structure (EXAFS) spectroscopies. The experimental results clearly show that the catalytic surface is not static, and the reaction atmosphere strongly affects the state of the surface and consequently the catalytic activity. The activity results show that the light-off temperature for CO oxidation increases with ex situ H2S addition. In situ IR results show a shift of the linear CO band, indicating a change in the bonding of adsorbed CO due to the presence of sulfur. Continuous co-feeding of 20 ppm of SO2 with the reactant mixture also increases the light-off temperature. Activity and IR results demonstrate that alumina acts as a sulfur storage reservoir. This delays the initial deactivation, but after extended time on stream in the presence of SO2 the activities of the silica- and alumina-supported catalysts are similar. The results indicate that sulfur poisoning is not only due to blocking of the sites where oxygen preferentially adsorbs, but also to modification of the Pt CO bonding.