The pH changes during the course of photocatalytic splitting of water significantly affect the band edges of the semiconductor and enhance or retard the rate of the splitting process. The effect of pH on photocatalytic activity in splitting of water over WO3, TiO2 (rutile) and NiO by using a monochromatic light source such as laser at 355 nm wavelength was studied for the first time. The mechanism of action of oxygen and metal ions (Fe3+ and Ag+) as electron capture agents was investigated on the basis of pH changes under illumination. It was observed that the pH changes measured during the course of reaction provides a deep insight of different processes occurring simultaneously in photocatalytic systems. (C) 2004 Elsevier B.V. All rights reserved.