Poisoning of a typical platinum-rhodium (Pt-Rh) automotive three-way catalyst (TWC) was determined as a function of lead (Pb), sulfur (S) and phosphorus (P) fuel levels, thermal aging and sulfur dioxide (SO2) content in the evaluation fuel. In laboratory studies catalysts were durability tested in pulse-flame reactors followed by flow-reactor activity measurements. Engine dynamometer-aged catalysts were evaluated on a slave vehicle.For Pt-Rh TWCs the activities for nitric oxide (NO), carbon monoxide (CO) and hydrocarbon (HC) conversions were poisoned by trace levels of 1-6 mg Pb/gal (0.3 - 1.6 g/m3). When the peak temperature in the aging cycle was increased from 730 to 870°C (1346 to 1598°F), the activities improved significantly. In an attempt to mimic the effect on TWCs of misfueling with Pb levels typical of commercially available leaded fuels, TWC activities were severely poisoned. Upon reverting to fuel containing trace Pb levels, the CO activity was restored, but the NO and HC activities were only partially recovered.The Pt-Rh TWCs were poisoned by the exhaust from fuel containing 0.03 wt % S, but not by-that from fuel containing up to 8 mg P/gal (2.1 g/m3). In the evaluation of thermally-aged catalysts, SO2 suppressed gross NO conversions and ammonia formation of Pt catalysts, but had little effect on Rh catalysts. Vehicle results for a TWC-only system indicated significantly lower HC, CO, and gross NOx conversions when the SO2 content of the exhaust gas was increased during rich operation with air/fuel fluctuations. In TWC systems followed by an oxidation catalyst, no effect from increased SO2 was observed on HC, CO or net NOx emissions.