The deactivation behavior of a Mo/Al2O3 and a NiMo/Al2O3 catalyst during the hydrodesulfurization of thiophene was investigated under gas-phase conditions. Four different deactivation mechanisms were considered: sintering and segregation of the active phase, blocking of the pore structure by coke deposition, poisoning of the active sites by coke and changes in the nature of the active sites induced by differences in pretreatment and reaction conditions. These mechanisms were investigated using quasi in situ XPS, analysis of the pore structure and coke content, and by measuring the HDS activity. During the thiophene HDS reaction, both catalysts show similar deactivation behavior: about 50% of the initial activity was lost during the first hours on stream. The extent of sintering/segregation and pore blocking was marginal and had no significant effect on the activity. A trend between the coke content and the deactivation was observed, but it is concluded that coke does not selectively poison the active sites. The main cause for the observed initial deactivation is the loss of sulfur from the active phase during the reaction. The number of sulfur groups and vacant sites on the catalysts is in equilibrium with the H2S/H-2 ratio of the gas phase. Exposure to H2S resulted in an increased initial activity or a (partial) reactivation, whereas exposure to H-2 caused a deactivation. It is concluded that the rate-determining step in the HDS of thiophene is catalyzed by structural sulfur; most probably, acidic SH groups facilitate the hydrogenation of the thiophenic ring. (c) 2006 Elsevier B.V. All rights reserved.