Photocatalytic decomposition of toluene was studied using commercially available TiO2 nanocatalysts (Degussa P25) under different ranges of relative humidity (RH). The toluene removal efficiency (sum of toluene adsorption, and partial and total oxidation) decreased with increasing RH, whereas the selectivity to CO2 in toluene oxidation enhanced. The enhancement in activity in terms of total oxidation at higher RH led to suppressing deactivation of TiO2 with time. On the basis of the humidity effect on photocatalytic reaction, deactivated TiO2 catalyst could recover their activity through the regeneration process consisting of exposure of the deactivated catalysts to the UV light under high RH conditions. The regenerated catalysts under high RH showed a higher total oxidation yield of toluene than that of the original catalysts due to the formation of more hydroxyl groups on TiO2 surface during the regeneration process. (C) 2013 Elsevier B.V. All rights reserved.