The catalytic activity and surface properties of nitrided 12.5% Mo/Al2O3 catalysts were studied using temperature-programmed reduction and diffuse reflectance FTIR and XPS spectroscopy. The activity of the nitrided Mo/Al2O3 catalysts was tested for the hydrodenitrogenation of carbazole. The catalytic activity of the 43.0, 58.0, 77.3, and 97.1% MoO3/Al2O3 catalysts was also determined for comparison. The MoO3/Al2O3 precursors with various molybdenum Loadings were nitrided by the temperature-programmed reaction with ammonia. The temperature-programmed reduction showed that the nitrided Mo/Al2O3 catalysts had a broad peak which was deconvoluted to six nitrogen peaks: four peaks due to nitrogen species adsorbed on the surface molybdenun species (MoO2, gamma-Mo2N, and Mo metal) and alumina and two peaks due to nitrogen release from the bulk molybdenum nitride. The infrared spectra of ammonia showed that the 1173 K nitrided catalyst was less acidic than the 773 Ir; nitrided catalyst but that its Lewis to Bronsted acidity ratio was 25 times higher. The 1173 K nitrided 12.5% Mo/Al2O3 catalyst had the highest TOF for the hydrodenitrogenation of carbazole, The XPS measurement showed that metallic Mo and Mo2+ were predominant in the 12.5% Mo/Al2O3 catalysts and led to the hydrogenation in the hydrodenitrogenation of carbazole.