Photocatalysis of a transparent aqueous WO3 sol was investigated for the color-fading of Orange II. Dependence of the degradation rate of Orange II on pH and ionic strength of the WO3 sol was explained in terms of the electrostatic interaction between Orange II and the WO3 colloid particles. The degradation rate follows the first-order kinetics with respect to the concentration of Orange II. It was demonstrated that the photogenerated electron in WO3 reduced O-2 effectively since the photochromism of the WO3 sol observed under nitrogen atmosphere did not occur under air-saturated condition. The photocatalytic activity of WO3 was lower than that of TiO2 by a factor of 0.5 and was enhanced by mixing with the TiO2 sol. The best photocatalytic performance for the degradation of Orange II was obtained by using the mixed sol with WO3:TiO2 molar ratio of 1:2. Similar enhancement in the photocatalytic activity was confirmed for the degradation of 4-chlorophenol. (C) 2013 Elsevier B.V. All rights reserved.