The photocatalytic degradation of metsulfurcin methyl (MTSM) on TiO2 in aqueous solutions is described in detail. Irradiation of the mixture TiO2/MTSM (1.0 g L-1/1.0 x 10-'mol L-1) within the wavelength range 300-450 nm leads to the complete degradation of the pesticide. Steady state experiments as well as pulse radiolysis, of MTSM aqueous solution,, clearly show the involvement of hydroxyl radicals. The rate constant of the reaction of MTSM with such reactive species leading to the adduct MTSM-(OH)-O-center dot wits evaluated to 2,7 x 10(9) mol(-1) L s(-1), The disappearance of MTSM is mainly due to three main routes: hydroxylation of the aromatic moiety of the pesticide, cleavage of the sulfonyl urea bridge and the demethylation of the methoxy group within the triazine part. The photoproducts were identified by using HPLC-MS-MS technique and a mechanism for their formation is proposed. The degree of mineralisation estimated by the difference between the initial total organic carbon amount and that obtained after prolonged irradiation was evaluated to 80%. The observed non-complete mineralisation is more likely owing to the formation of cyanuric acid product from the oxidation of the triazine part of the pesticide, Efficient photocatalytic disappearance of MTSM was also found when natural water and solar irradiation were used. (c) 2005 Elsevier B.V. All rights reserved.