The influence of NO on the performances of VOx/TiO2, VOx-WOx/TiO2, and VOx-MoOx/TiO2 catalysts is investigated in the combustion of chlorobenzene. NO proves to induce an increase in the chlorobenzene conversion. However, this happens only if O-2 is present and is maximized when the catalyst contains W or Mo. The suggested mechanism for the effect is: 1) NO is oxidized to NO2 mainly on WO, and MoOx; 2) afterward, NO2 assists O-2 in the reoxidation step (as described by Mars-van Krevelen) of the VOx phase, thus speeding up the oxidation cycle, which macroscopically corresponds to the increase in chlorobenzene conversion. (c) 2005 Elsevier Inc. All rights reserved.