The photooxidation of the fungicide metalaxyl in aqueous solution has been examined. Irradiation at 290 nm in the presence of hydrogen peroxide and titanium dioxide resulted, respectively in 29 and 84% substance transformation in 2.5 h. The degradation approximately followed first order kinetics. Irradiation under artificial sunlight in the presence of H2O2 and TiO2 resulted in 17 and 80% degradation of the chemical. Irradiation of metalaxyl in the presence of H2O2 leads to the oxidation of the molecule and in the presence of TiO2 deacylation from the molecule takes place.