In order to understand the role of vanadium and its interaction with molybdenum atoms in the catalysts used for the hydrotreatment of residues, mixed heteropolyanion (HPA) precursors PMo(12-x)VxO40(3+x)-have been prepared and deposited by incipient wetness impregnation on an alumina support. The evolution of the compounds in the impregnating solutions till their deposition on the support has been analysed using various spectroscopic techniques such as Raman, P-31 and V-51 NMR, XPS as well as TEM. Activity measurements on model molecules (toluene and cyclohexane) and on real feedstock (vacuum residue) have been performed after activation step (ex situ sulphidation). Hydrogenation (HYD), isomerisation (ISOM), hydrodemetallation (HDM), hydrodesulphurisation (HDS), hydrodeasphaltenisation (HDAsC7) were assessed. During tests (2 h) on a Safaniya vacuum residue feed, interesting HDM activity was observed compared to reference promoted and unpromoted catalysts (NiMo and Mo/Al2O3). It can be concluded that the use of vanadium in the impregnating solution may promote HDM and HDAsC7 (increase of vanadium removal rate around 15% compared to Mo-catalyst without vanadium). Furthermore HDM and HDAsC7 catalytic performances turned out to be close to the ones of NiMo catalyst. The increase in HDM/HDAsC7 activity may be related to the improvement of hydrogenation and/or isomerisation, the latter being improved by the presence of vanadium. (C) 2010 Elsevier B.V. All rights reserved.