Adsorption of SO2 on a Pt/C catalyst typically used in proton exchange-membrane fuel cells (PEMFCs) has been investigated by temperature programmed desorption (TPD). SO2 concentrations in N-2 were varied from 5 ppm to 1% (vol) and adsorption isotherms were determined at 25, 50, and 80 degrees C. Oxygen assisted (O-assisted) desorption experiments (i.e., successive TPD experiments following exposure to room temperature O-2 after the first TPD event) produced an additional SO2 peak at a temperature higher than the initial SO2 peak. These two types of SO2 adsorption were identified as weakly adsorbed SO2 species desorbed between 140 and 200 degrees C, depending on concentration, and a strongly adsorbed, dissociated species. For the strongly adsorbed, dissociative species, O-18(2) isotope introduction during O-assisted desorption yielded ratios of 50%, 36%, and 14% for SO2 masses of 64, 66, and 68, respectively. The activation energy and kinetic constant of desorption are reported for weakly adsorbed SO2 at 1% and 20 ppm SO2 using the Polanyi-Wigner equation.