The preparations of pure and mixed vanadium-nickel sulphides were carried out by the homogeneous precipitation method (under inert atmosphere). Thiophene HDS, pyridine HDN, toluene hydrogenation and vanadyl porphyrin HDP, were carried out as catalytic tests. The catalysts were also characterized by XPS, TPR, BET surface area and XRD. These solids, after sulphidation procedure, exhibit a synergetic effect, the importance of which depends on the hydrotreating reaction. The product distributions obtained in the thiophene hydrodesulphurization and the pyridine hydrodenitrogenation and the activity observed for toluene hydrogenation suggest that the synergy is mainly related to the hydrogenation step. The cause of this synergy could tentatively be assigned to an electronic transfer between the two metallic components in the mixed sulphides. (C)2000 Elsevier Science B.V. All rights reserved.