The degradation of the pesticide metsulfuron methyl induced by excitation of iron(III) aqua complexes was studied at 365 nm. The process involved the formation of hydroxyl radicals and was shown to depend on Fe(OH)(2+) concentration. The second order rate constant of the reaction of metsulfuron methyl and hydroxyl radical was evaluated by laser flash photolysis experiments to k = 3.5 x 10(9) mol(-1) L s(-1). Various products were formed. Among them 2-amino-4-methoxy-6-methyl-1,3,5-triazine, 2-(carbomethoxy) benzenesulfonamide and hydroxylated derivatives represent the major photoproducts. Compounds arising from the cleavage of the triazine moiety were also found. After prolonged irradiation, a total disappearance of metsulfuron methyl and a complete mineralization of the solution were obtained.