The adsorption and reactions behaviors of thiophene on reduced passivated and nitrided Mo2N/gamma-Al2O3 catalysts have been studied by in situ FT-IR spectroscopy at temperatures from room temperature to 673 K. Thiophene mainly adsorbs on the cus Mo sites of the two catalysts surfaces, and its adsorption mode is determined to be eta(1)(S). Thiophene alone shows no reaction on the two catalysts even at 673 K, while in the presence of H-2, it becomes reactive initially at 573 and 373 K on reduced passivated and nitrided Mo catalysts, respectively, producing a surface a-bonded butene species that is confirmed by the adsorption of 1-butene, cis- and trans-2-butenes, and 1,3-butadiene on the catalysts. Adsorption of CO clearly shows that the catalyst surfaces become sulfided completely with a treatment of thiophene/H-2 at temperatures higher than 573 K. The surface sulfidation together with the sigma-bonded C-4 species may play a role in the deactivation of the nitride catalysts for the HDS of thiophene. The high reactivity of thiophene toward nitrided Mo catalyst in the initial stage of thiophene HDS is ascribed to the high density of surface sites and unique electronic property of nitrided Mo for the activation of both H-2 and thiophene.