A Mo/Al2O3 and a NiMo/Al2O3 catalyst were subjected to the gas-phase hydrodesulfurization (HDS) of thiophene. The mechanism of catalyst deactivation was investigated. Sintering or modification of the active phase and blocking of the active sites or the pore structure by coke deposition were considered as possible causes. The freshly sulfided and the spent catalysts were characterized by quasi in situ TEM, IR spectroscopy on adsorbed CO, total carbon analysis, N-2 chemisorption and Hg porosimetry. For the NiMo/Al2O3 catalyst the main cause for deactivation is loss of sulfur during the reaction. This process is fully reversible by H2S/H-2 treatment. In contrast coke deposition on the active sites is a major cause for deactivation of the Mo/Al2O3 catalyst. (C) 2003 Elsevier B.V. All rights reserved.