We studied Pd/Rh automotive catalysts containing varying amounts of ceria, using either high-sulfur or low-sulfur fuel. The effects of sulfur strongly depended on whether catalysts were tested under reducing conditions or at "stoichiometry," at which oxidants and reductants are balanced. Under reducing conditions, sulfur is present as H2S, which strongly poisons metal surfaces. In particular, this affects activity for the oxidation of hydrocarbons. The role of ceria is complex. NOx conversion is sharply improved by ceria, especially in combination with rhodium. However, under certain conditions, ceria, due to its ability to store and release sulfur, can be shown to increase the negative impact of sulfur.