The photocatalytic degradation of methyl parathion in aqueous solutions, using two different photocatalysts (TiO2 and ZnO) has been investigated. The degradation of methyl parathion follows first order kinetics according to the Langmuir-Hinshelwood model. Complete degradation is achieved within 45 or 150 min when treated with illuminated TiO2 or ZnO, respectively. It was observed that the initial rate increases linearly with an increase of the amount of catalyst up to a level where it reaches a plateau corresponding to the optimum of light absorption. The addition of an oxidant (K2S2O9) leads to an increase in the rate of photooxidation. Moreover, illuminated TiO2 suspensions were proved to be more effective in mineralizing the insecticide compared to ZnO suspensions. Measurements of phosphate, sulfate and nitrate ions gave valuable information about how this process is achieved. Addition of the oxidant enhances mineralization for both photocatalytic systems. Up to eight byproducts were identified by GC-MS technique during the photocatalytic degradation of the insecticide that proceeds via oxidation, hydroxylation, dealkylation and hydrolysis of the ester group reaction pathways. Finally, the toxicity of the treated solution was reduced only in the presence of TiO2, while ZnO suspensions appear to increase the toxicity due to photo-dissolution of ZnO releasing zinc in the treated solution. (C) 2007 Elsevier B.V. All rights reserved.