In the present study, the photocatalytic degradation of an organophosphorous insecticide, dichlorvos, using two different photocatalysts (TiO2 and ZnO) has been investigated. The blank experiments for either illuminated dichlorvos solution or the suspension containing TiO2 or ZnO and dichlorvos in the dark showed that both illumination and the catalyst were necessary for the destruction of the pesticide. Dichlorvos disappearance is achieved in similar to 20 min when treated with illuminated TiO2 and in similar to 120 min with illuminated ZnO. The degradation of the organic molecule follows first-order kinetics according to the Langmuir-Hinshelwood model. It was observed that the initial rate increases linearly with an increase in the amount of catalyst up to a level where it reaches a plateau. Temperature and pH also affect the rate of the reaction. The addition of an oxidant (H2O2 or K2S2O8) to the TiO2 suspensions leads to an increase in the rate of photooxidation. On the contrary, the addition of hydrogen peroxide in ZnO suspensions caused a decrease in the reaction rate. Moreover, illuminated TiO2 suspensions were proved to be quite effective in mineralizing dichlorvos. Measurements of chlorine and phosphate ions gave valuable information about how this process is achieved. On the other hand, ZnO suspensions could not lead in complete mineralization of the insecticide. Addition of the oxidants enhances mineralization for both photocatalytic systems. Finally, toxicity measurements showed that toxicity increases during treatment independently from the type of catalyst or the oxidant that has been used, indicating the inefficient detoxification capacity of the processes. (c) 2005 Elsevier B.V. All rights reserved.