A series of gamma-Al2O3-supported molybdenum and nickel sulfide catalysts was prepared in order to investigate the role of the nickel promoter and the effect of the sulfur feed compounds on the hydrodenitrogenation (HDN) performance of Ni-Mo/gamma-Al2O3 catalysts. The reactions performed were pyridine HDN, thiophene HDS, HDN in the presence of thiophene, and HDN in the presence of H2S. The catalysts were characterized using laser Raman spectroscopy, X-ray diffraction, X-ray photoelectron spectroscopy, BET surface area measurement, and temperature-programmed desorption of HsS, thiophene, and pyridine. The results of the reaction studies and characterization experiments are combined with earlier reports in the literature to propose an explanation of the role of nickel and gas phase sulfur compounds in pyridine HDN network.