The hydrodesulfurization (HDS) of thiophene was carried out over mildly deactivated NiMo/alumina catalysts used for the hydrotreatment of coal-derived liquids. Arrhenius plots were obtained covering a wide temperature range, 180-540-degrees-C. The observed rate constant is found to level off at the highest temperatures. This phenomenon cannot be explained by further catalyst deactivation, catalyst desulfiding, external mass transfer control, formation of an inert external shell or thermodynamic equilibrium for the HDS of thiophene. A modified two-step reaction mechanism is suggested to explain these results. Step 2 has a higher activation energy and controls the reaction rate at lower temperatures. Step 1 has a low activation energy and controls the rate at the highest temperatures. The proposed mechanism successfully predicts the experimental observations. Increasing deactivation affects the effective diffusivity to a greater extent than it does the intrinsic reaction rate constants.