The effect of adding different amounts of fluoride to a gamma-alumina used as support of NiW/Al2O3 catalysts was investigated using X-ray diffraction, diffuse reflectance spectroscopy (DRS), infrared spectroscopy of adsorbed NO (IR-NO), X-ray photoelectron spectroscopy (XPS), surface acidity, specific surface area and catalytic activity measurements. The catalytic activity was evaluated by two model reactions, namely thiophene hydrodesulfurization (HDS) and pyridine hydrogenation (HDN), in the temperature range 573-623 K using a flow system operated at 2 MPa (HDS) or 3 MPa (HDN) total pressure. DRS results of the oxidic precursors suggested that the effect of fluoride incorporation to the alumina support is that it increases slightly the octahedrally coordinated fractions of Ni and W. The XPS results of the sulfided catalysts showed that the dispersion of the Ni and W phases decreased progressively with increasing fluoride content. This technique also showed that these phases were only partially sulfided under the present experimental sulfiding conditions, and that the extent of sulfidation of such phases apparently tends to decrease slightly with increasing fluoride content, in contrast with the results suggested by the IR-NO spectra. The sulfur content of sulfided samples, as measured by chemical analysis, revelated that the sulfidation degree increased only slightly with increasing fluoride content. The activity for thiophene HDS was almost invariant with fluoride addition, but the subsequent hydrogenation of the intermediate products (butenes) was found to decrease linearly with the fluoride content. Possible interpretations for these changes in activity and selectivity, which reflect changes in the relative distribution of the different active sites for the two reactions, are discussed in relation with the structural variations of the active phases described above. In contrast, the activity for pyridine HDN increased considerably with the incorporation of small amounts of fluoride; this increase is mainly attributed to the increase in surface acidity which was observed with fluoride addition.