The electrochemical treatment of solutions containing 4-chloro-2-methylphenoxyacetic acid (MCPA) concentrations up to 370 ppm in the pH range 2.0-6.0 and at 35 degreesC has been studied by peroxi-coagulation. This method yields a rapid depollution at low applied currents due to the oxidation of the herbicide and its products with hydroxyl radicals produced from Fenton's reaction between Fe2+ and H2O2 electrogenerated by the corresponding Fe anode and O-2-diffusion cathode. A faster degradation is feasible using a photoperoxi-coagulation treatment under UV irradiation of solutions, because of the production of more hydroxyl radicals from the photo-Fenton reaction. Both methods are more efficient at pH 3.0 without pH regulation, mainly for MCPA concentrations less than or equal to186 ppm, although their optimum pH is 4.0 when the solution pH is regulated. Mineralization predominates over the coagulation of products with the Fe(OH)(3) precipitate formed being favored as the herbicide concentration increases under pH regulation and for MCPA concentrations greater than or equal to186 ppm without pH regulation. The MCPA decay follows a pseudo first-order reaction, with a similar rate constant for both methods. 4-Chloro-o-cresol, methylhydroquinone and methyl-p-benzoquinone are identified as aromatic intermediates by reverse-phase chromatography. Oxidation of chlorinated products is accompanied by the release of chloride ion, which is mainly accumulated in the medium. Ion-exclusion chromatography allows detection of the formation of carboxylic acids such as glycolic, glyoxylic, formic, malic, maleic, fumaric and oxalic acids, during degradation. The evolution of all intermediates with electrolysis time is discussed and a reaction scheme involving all oxidation products is proposed.