A series of Re(x)/gamma-Al2O3 catalysts containing 1.69-6.76 wt.% Re2O7 were prepared by wet impregnation and characterized using X-ray diffraction (XRD), BET surface area, diffuse reflectance spectroscopy (DRS), electrophoretic migration (EM), X-ray photoelectron spectroscopy (XPS) and surface acidity techniques. The catalysts were tested for simultaneous hydrodesulfurization (HDS) and hydrodenitrogenation (HDN) of commercial gas oil under typical industrial hydrotreating conditions. In general, the activities for both HDS and HDN reactions increased almost linearly with increasing Re loading, reaching maxima at about 0.5 Re atoms nm(-2) and then leveled off. These activity trends resemble that observed for the Re dispersion as measured by EM (in the oxided state) and XPS (in the sulfide state), which resulted in a monolayer-type deposition up to about 0.5 Re atoms nm(-2) and then in multilayers or small aggregates. The magnitude of the catalytic effect was, however, different for HDS and HDN, and strongly depends on the Re content and reaction temperature. This led to a marked increase in the HDN/HDS selectivity with decreasing temperature (values above 1.5 at 325 degreesC), due to the large differences in the apparent activation energies of HDS and HDN found for all catalysts. A gradual increase in the HDN/HDS selectivity with increasing Re loading was also found. Such selectivity increase was ascribed to the parallel increase of catalyst acidity observed.