Catalytic reforming of liquid hydrocarbon fuels is challenging due to potential deactivation via carbon deposition and sulfur poisoning. To gain a better understanding of the effect of sulfur on the deactivation of Ni/Ce0.75Zr0.25O2 catalysts, isooctane conversion to syngas was studied in the presence of small amounts of thiophene under various O/C and H2O/C ratios representing steam reforming, partial oxidation, and autothermal reforming conditions. It was found that depending on the reaction conditions, thiophene underwent different degrees of desulfurization, leading to the formation of H2S. Under reaction conditions leading to nearly complete conversion of thiophene to H2S, the nickel catalyst lost only a small amount of its initial activity, but then maintained stable performance over longer times on stream. In contrast, reaction conditions under which thiophene emerged unconverted from the reactor led to severe and continued deactivation of the catalysts. Furthermore, co-feeding thiophene with isooctane caused significant increases in the temperature profile of the reactor and an increased amount of olefins were seen as products of the reaction, indicating that sulfur deactivated primarily catalyst sites responsible for endothermic steam reforming reactions, while having less impact on exothermic partial oxidation reactions. Controlling the reaction conditions in such a way as to generate sufficient hydrogen concentrations in the catalyst bed for effectively desulfurizing thiophene to H2S appears to be the key to maintain stable catalytic activities in the presence of sulfur compounds. (C) 2010 Elsevier Inc. All rights reserved.