Two series of NiRe catalysts supported on γ-Al 2 O 3 and activated carbon having constant renium content and various Ni/Re atomic ratios were prepared to investigate the promoting action of Ni on Re in the simultaneous hydrodesulfurization and hydrodenitrogenation of gas oil at 325, 350 and 375 °C. On both supports, the dependence of activities showed initially an increase and subsequent decrease with increasing Ni content, with optimum Ni/Re atomic ratios of ∼0.94 for Ni( x )Re/Al 2 O 3 catalysts and ∼1.25 for Ni( x )Re/C catalysts. Two series of NiRe catalysts supported on γ-Al 2 O 3 and activated carbon having constant wt.% Re 2 O 3 content (3.51 and 1.97, respectively) and various Ni/Re atomic ratios were prepared to investigate the promoting action of Ni on sulfided Re catalysts in the simultaneous hydrodesulfurization (HDS) and hydrodenitrogenation (HDN) of gas oil. The catalysts were characterized using textural analysis, X-ray diffraction, diffuse reflectance spectroscopy, energy-dispersive X-ray analysis and X-ray photoelectron spectroscopy. On both Ni( x )Re/Al 2 O 3 and Ni( x )Re/C catalysts, the dependence of HDS and HDN activities showed initially an increase and subsequent decrease with increasing Ni content, with optimum Ni/Re atomic ratios of ∼0.94 for Ni( x )Re/Al 2 O 3 catalysts and ∼1.25 for Ni( x )Re/C catalysts. The decline in activity was attributed to a reduction in Re dispersion and a low Ni sulfidation for the alumina-supported catalysts, and to a separate location of Re and Ni on the support surface, as well as to an incomplete Ni sulfidation for the carbon-supported catalysts. The synergetic effect in activity observed between Ni and Re was slightly higher for HDS than for HDN, especially at low reaction temperature (325 °C) and on carbon-supported catalysts. On the basis of these promotional effects on catalytic activities and HDN/HDS selectivity, and the characterization results, the nature of the promotion observed on NiRe supported catalysts was discussed.