The addition of water during the oxidation of m-dichlorobenzene over a 3.6 wt.% V2O5/TiO2 catalyst resulted in an unusual bimodal activation/deactivation behavior at different temperatures. Catalyst activity increased at temperatures up to 300 degrees C, with a maximum observed at 270 degrees C, while it decreased at temperatures above 300 degrees C with a minimum observed at 350 degrees C. This effect was proportional to the concentration of water in the feed stream. At lower reaction temperatures (i.e., <270 degrees C), the addition of water resulted in a lower concentration of surface chloride and carbonaceous deposits. In contrast, at higher reaction temperatures (i.e., >350 degrees C), the presence of water did not affect the surface carbon and chlorine contents, but resulted in the deactivation of the catalyst due to the competitive adsorption on the active sites. Characterization of used catalyst samples indicated no changes in bulk characteristics, suggesting that the effect of water was limited to only reversible changes taking place on the catalyst surface. Finally, the presence of water also favored the formation of partial oxidation products, although the overall selectivity towards such products still remained low (i.e., yields below 3%). (C) 2010 Elsevier B.V. All rights reserved.