In spite of the recognized importance of their industrial application, the questions regarding catalytic phenomena involved in hydrodenitrogenation (HDN) reactions still remain unanswered. In this study, gamma-Al2O3-supported Ni, Mo, and bimetallic Ni-Mo catalysts were used for investigating the pyridine HDN and the,simultaneous pyridine HDN/thiophene HDS reactions. The purpose of this study was to investigate the role of Ni as a promoter in the Ni-Mo catalyst system and to examine the effect of the presence of sulfur-containing compounds in the gas phase on HDN reaction. The results showed the C-5 yield to be a strong function of the Mo loading regardless of whether Ni was added to Mo or not, whereas the piperidine yield showed no dependence on Mo loading. The findings of this study, when combined with the catalyst characterization results, lead us to suggest that Mo active species promote hydrogenolysis of piperidine while Ni-Mo species enhance the hydrogenation of pyridine. The experimental results also show the enhancement effect of the presence of sulfur-containing compounds on HDN, especially when Ni is combined with Mo.