The effects of Re content and catalyst preparation on the surface structure of two series of Re/gamma-Al2O3 catalysts, prepared by one-step (Re(x)-II) and successive-step impregnation (Re(x)-II) methods, and their catalytic behaviour in the HDS and HDN of gas oil were studied. Characterization data by XRD, Raman spectroscopy and XPS revealed that the structure of Re on Re(x)-I and Re(x)-II catalysts in their oxidic state was very similar: Re in oxided state was present as two slightly different isolated ReO4- monomeric species coordinated to the alumina surface in all the range of Re content Studied, but the Re coverage was slightly lower on Re(x)-I than on Re(x)-II catalysts. Upon sulfidation, the dispersion of Re decreased significantly and left to be monomolecularly dispersed above 0.5 Re at. nm(-2), especially for the Re(x)-I catalysts. causing therefore that the HDS and HDN activities of the two catalyst series reached maximum values at a Re content of about 0.5 at. nm(-2) and then decreased. The higher activity for both HDS and HDN reactions observed for the Re(x)-II catalysts are consistent with their higher dispersion of the Re sulfided phase. Both Re(x)-I and Re(x)-II catalysts showed exceptionally high HDN/HDS selectivity values, which were slightly higher for the Re(x)-II catalysts due to their slightly higher concentration of acid sites in the oxidic state. (c) 2004 Elsevier B.V. All rights reserved.