The photocatalytic degradation of dimethoate, an organophosphorous pesticide, has been investigated using TiO 2 and ZnO as catalysts. The degradation kinetics were studied under different conditions such as substrate and photocatalyst concentration, temperature, pH and addition of oxidants. The degradation rates proved to be strongly influenced by these parameters. Comparison between the two catalysts showed that TiO 2 is more efficient to the disappearance of dimethoate and follows first-order kinetics. DOC measurements were carried out in order to evaluate the degree of mineralization. Both catalysts were unable to mineralize dimethoate, but the addition of oxidants improved the efficiency of the processes. Measurements of phosphate, ammonium and sulphate ions gave valuable information about how this process is achieved. Toxicity measurements showed that only the addition of hydrogen peroxide in the illuminated titanium dioxide suspension was able to lead to a complete detoxification of the solution.