The relationship between the activity and surface molybdenum species of nitrided 12.5% MoO3/Al2O3 was studied in the hydrodenitrogenation (HDN) of carbazole at 573 K and 10.1 MPa total pressure. The surface molybdenum species were determined by the desorption of nitrogen gas during TPD. The surface area of NH3-cooled Mo/Al2O3 nitrided at 773 and 1173 K was decreased by 8% and 61% from 245 m(2) g(-1) of the fresh MoO3/Al2O3, respectively. The NH3-cooled Mo/Al2O3 catalysts had slightly higher surface area than the He-cooled catalysts. The HDN rate increased with increasing nitriding temperature in the HDN of carbazole on the nitride catalysts. The NH3-cooled Mo/Al2O3 catalysts nitrided at 1173 K were the most active in carbazole HDN and the He-cooled catalyst nitrided at 773 K was the least.