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, γ-Al 2 O 3 -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