Adsorption of SO2 in the presence of O-2 on Pt/C catalysts often used as electrocatalysts has been investigated by temperature programmed desorption (TPD) and X-ray photoelectron spectroscopy (XPS). The amounts of SO2 adsorption on Pt/C in the presence of O-2 were much higher than those in the absence of O-2 (SO2-N-2) and from the carbon support (Vulcan XC-72) alone. Adsorption is dependent on oxygen concentration over the range 0-20% but reaches saturation at 20% O-2. The spillover of SO2 from Pt to the carbon support has been proposed for 10, 20, and 40% Pt loadings, characterized by desorption temperatures of approximately 150 and 260 degrees C for SO2 adsorbed on Pt and carbon, respectively. Adsorbed Pt-S, C-S, C-SOx, and Pt-SO4 species were identified by XPS as S-containing species on both Pt and carbon. Both TPD and XPS indicate that the carbon support plays a major role in SO2 adsorption, primarily as SOx (x = 3, 4). The bonding of S and SOx on the carbon support was strong enough that back diffusion to the Pt surface did not occur.