The 12.5% MoO3/Al2O3 catalysts were nitrided by the temperature-programmed reaction with ammonia at various temperatures. The molybdenum species of the nitrided Mo/Al2O3 catalysts were studied, based on X-ray photoelectron spectroscopy (XPS), temperature-programmed surface reaction (TPSR), and diffuse reflectance FTIR spectroscopy. The activity of the nitrided catalysts was measured for the hydrodesulfurization of dibenzothiophene at 573 K and a total pressure of 10.1 MPa. The TOF (based on CO adsorption) increased; the 1173 K nitrided catalyst was the most active. From the XPS analysis, the molybdenum oxidation states of the catalysts were widely distributed from Mo6+ to MO0. Mo2+ and Mo-0 were predominant on the surface of the 1173 K nitrided catalyst. The TPSR results also indicated that the 1173 K nitrided catalyst consisted of metallic molybdenum. FTIR spectroscopy of chemisorbed pyridine on the catalysts at 573 K showed that the Lewis acid sites (1450 cm(-1)) on the 973 K nitrided catalyst were present more than on the 773 and 1173 K nitrided catalysts, due to the abundance of Mo4+ ions. The distribution of molybdenum oxidation states of the nitrided Mo/Al2O3 catalysts was related to the HDS activity. Metallic Mo and Mo2+ are the most active species and are responsible for the hydrodesulfurization of dibenzothiophene.