The photocatalytic degradation of a small concentration of an organo-phosphorous (OP) insecticide phosphamidon, in water, on ZnO and TiO2 is investigated. Of the two semiconductor oxides, TiO2 is found to be more effective as a photocatalyst for this reaction. Several factors such as concentration of phosphamidon, pH of the system, catalyst loading and presence of anions are found to influence the degradation rate. The reaction follows apparent first-order kinetics, though at higher concentrations, there is a reduction in the order of the reaction. There is a simultaneous formation and decomposition of H2O2 in the system, resulting in a periodic increase and decrease in its concentration. The catalyst TiO2 is effective for the degradation of phosphamidon in direct sunlight and thus opens the possibility of solar decontamination of wastewater containing small amounts of OP pesticides. (C) 2003 Elsevier B.V. All rights reserved.