Two series of NiRe catalysts supported on gamma-Al2O3 and activated carbon having constant wt.% Re2O3 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 hydro-desulfurization (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/Al2O3 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 similar to 0.94 for Ni(x)Re/Al2O3 catalysts and similar to 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 degrees 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. (c) 2007 Published by Elsevier B.V.