A series of NiW/gamma-Al2O3 catalysts (20 and 30 wt% W and 1-5 wt% Ni) have been prepared and studied by TPR and XPS. HDS activity has been tested in the thiophene conversion. The effect of Ni and W loadings on the formation of different structures is presented. In the calcined catalysts several phases coexist, concentrations of which depend on the Ni/(Ni+W) atomic ratio. The Ni synergistic effect in the HDS reaction is confirmed by the increase in the HDS activity (similar to 10-15 times). This effect is ascribed to the formation of an active NiWS phase of high dispersion from the mixed NiW oxide precursors. At higher Ni/Ni+W ratio a redistribution of active components in additional amount of NiWS phase during sulfidation is suggested.