Commercial WO3 powder was annealed in air at four different temperatures and characterized by XRD and BET. The samples were used for the photooxidation of H2O to O-2 under visible light irradiation (lambda > 420 nm) in the presence of IO3 and the evolved gases were analyzed by gas chromatography. The results showed that the WO3 photocatalyst of monoclinic phase, which was obtained by annealing at 750 degrees C for 4 h, displayed the best activity in terms of O-2 evolution among all the samples. Moreover, the activity was also found to be slightly affected by the grain size of the WO3 samples. (C) 2009 Elsevier B.V. All rights reserved.