Pelleted and monolith three-way catalysts were treated in a laboratory reactor under conditions closely simulating automobile exhaust and then characterized using x-ray photoemission and temperature programmed desorption techniques to determine relative amounts and oxidation states of sulfur stored by the catalyst. Sulfur originating as SO2 in the feed was stored on the support component of both catalysts in the form of adsorbed sulfates and sulfites and on the noble metals in the form of elemental sulfur. The monolith catalyst stored a greater amount of sulfur and equilibrated more rapidly with the sulfur content in the feed than did the pelleted catalyst.Operation in a rich environment removes sulfur from the support components, while operation in a lean environment removes sulfur from the noble metal surfaces. This behavior is consistent with the observation that sulfur inhibits three-way activity in a rich environment. Sulfur poisoning becomes less important at operating temperatures above 700°C.