A series of Re-containing catalysts supported on activated carbon, with Reloading between 0.74 and 11.44 wt.% Re2O7, was prepared by wet impregnation and tested in the simultaneous hydrodesulphurisation (HDS) and hydrodenitrogenation (HDN) of a commercial gas oil. Textural analysis, XRD, X-ray photoelectron spectroscopy (XPS) and surface acidity techniques were used for physicochemical characterisation of the catalysts. Increase in the Re concentration resulted in a rise in the HDS and HDN activity due to-the formation of a monolayer structure of Re and the higher surface acidity. At Re concentrations >2.47 wt.% Re2O7 (0.076 Re atoms nm(-2)) the reduction in the catalytic activity was related to the loss in specific surface area (BET) due to reduction in the microporosity of the carbon support. The magnitude of the catalytic effect was different for HDS and HDN, and depended strongly on the Re content and reaction temperature. The apparent activation energies were about 116-156 kJ mol(-1) for HDS and 24-30 kJ mol(-1) for HDN. This led to a marked increase in the HDN/HDS selectivity with decreasing temperature (values >3 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 increased Re loading was also found and related to the observed increase of catalyst acidity. The results are compared with those obtained for a series of Re/-Y-Al2O3 catalysts.