The reactions of both thiophene and H₂S on Mo₂C/Al₂O₃ catalyst have been studied by in situ FT-IR spectroscopy. CO adsorption was used to probe the surface sites of Mo₂C/Al₂O₃ catalyst under the interaction and reaction of thiophene and H₂S. When the fresh Mo₂C/Al₂O₃ catalyst is treated with a thiophene/H₂ mixture above 473 K, hydrogenated species exhibiting IR bands in the regions 2800–3000 cm⁻¹ are produced on the surface, indicating that thiophene reacts with the fresh carbide catalyst at relatively low temperatures. IR spectra of adsorbed CO on fresh Mo₂C/Al₂O₃ pretreated by thiophene/H₂ at different temperatures clearly reveal the gradual sulfidation of the carbide catalyst at temperatures higher than 473 K, while H₂S/H₂ can sulfide the Mo₂C/Al₂O₃ catalyst surface readily at room temperature (RT). The sulfidation of the carbide surface by the reaction with thiophene or H₂S maybe the major cause of the deactivation of carbide catalysts in hydrotreating reactions. The surface of the sulfided carbide catalyst can be only partially regenerated by a recarburization using CH₄/H₂ at 1033 K. When the catalyst is first oxidized and then recarburized, the carbide surface can be completely reproduced.