Rotating disk electrode (RDE) methods are used to quantify the poisoning of the oxygen reduction reaction (ORR) at Pt catalysts for PEMFCs by SO2. With the addition of 50 ppm of SO2, the Pt loses 17% of its mass activity, however its Tafel slope remains unchanged. With 200 ppm SO2, the catalyst activity drops by 52% and its Tafel slope changes from 129 to 186 mV dec-1 at potentials <0.83 V. Levich-Koutecky analysis shows significant deviation from mass-transport controlled kinetics at high concentrations (200 ppm) of SO2, even at high overpotentials, demonstrating poisoning of the Platinum/Vulcan Carbon (Pt/VC) catalysts. The analyses show that SO2 poisoning of Pt at higher potentials (> 0.85 V) is mainly due to blocking of Pt sites by adsorbed SO2, and is more severe at lower potentials (< 0.83 V) due to stronger adsorption that alters the electronic state of the Pt.