The chemical oxidation of four selected phenylurea herbicides (linuron, chlortoluron, diuron, and isoproturon) was studied by means of the Fenton system. The influence of the initial concentrations of hydrogen peroxide and ferrous ions, the pH and the type of buffer (perchloric acid/perchlorate, acetic acid/acetate, or phosphoric acid/phosphate) was established according to the degradation levels obtained. In the kinetic study, the general decomposition reaction was divided into two stages with different reaction rates, which was justified by considering the whole reaction mechanism for this system. In this kinetic study, a competition kinetics model, which used p-chlorobenzoic acid as a reference compound, was applied for the evaluation of the rate constants for each reaction between the herbicides and the hydroxyl radical. The proposed values for these rate constants are: 7.5 x 10(9) L mol(-1) s(-1) for chlortoluron, 5.6 x 10(9) L mol(-1) s(-1) for linuron, 7.1 x 10(9) L mol(-1) s(-1) for diuron and 5.7 x 10(9) L mol(-1) s(-1) for isoproturon. Finally, some experiments with the photo-Fenton system reveal increases in the decomposition levels of the herbicides, due to additional generation reactions of hydroxyl radicals. (c) 2007 Society of Chemical Industry