The influence of H2O on the performances of VOx/TiO2, VOx-WOx/TiO2 and VOx-MoOx/TiO2 catalysts is investigated in the combustion of chlorobenzene. H2O proves to influence in opposite manners the chlorobenzene conversion depending on its concentration and on the kind of catalysts. The overall influence of water is the sum of three effects, two negative: (1) the reduction of the vanadium phase dictated by its reducibility, itself influenced by the presence of WOx or MoOx, (2) the decrease of the number of strong Bronsted acid sites involved in the adsorption of the chlorobenzene and one positive (3) the retrieval of chlorine species from the Surface through the production of HCl. (c) 2005 Elsevier B.V. All rights reserved.