During combustion, organosulfur compounds typically contained in gasoline are converted to SO2. Over automotive emission control catalysts, the SO2 can be converted to other sulfur compounds such as H2S,COS, and H2SO4. The chemistry of sulfur over catalysts is a function of A/F as well as catalyst composition.Exhaust emission control catalysts are also poisoned by exhaust SO2. The present study probes the extent of poisoning as a function of A/F, fuel sulfur levels and noble metal composition. The effect of fuel sulfur levels (14-6000 ppm) during aging and evaluation of platinum/rhodium, palladium/rhodium and palladium-only three-way control catalysts was evaluated. Performance measurements are reported for both engine dynamometer and vehicle systems.