The mineralization of aqueous solutions of the herbicide 4-chloro-2-methylphenoxyacetic acid (MCPA) in 0.05 M Na2SO4 with adjusted pH 2.0-6.0 using H2SO4 has been studied by an UV-assisted cathodic Fenton treatment such as photoelectro-Fenton at low current and at 35degreesC, using an undivided cell with a Pt anode and an O-2-diffusion cathode. This method allows the complete depollution of solutions of pH 3.0 with an herbicide concentration of up to 380 mg L-1 and 1 mM Fe2+. This is feasible by the high concentration of oxidizing hydroxyl radicals produced in the medium by the Fenton's reaction between added Fe2+ and H2O2 electrogenerated by the cathode, along with photolysis of some products. Comparative treatment by a general cathodic Fenton method such as electro-Fenton only yields 70-75% of mineralization due to the formation of hardly oxidizable products. MCPA is much more slowly degraded by anodic oxidation in the absence and presence of electrogenerated H2O2. The herbicide decay always follows a pseudo first-order reaction. Aromatic products such as 4-chloro-o-cresol, methylhydroquinone, and methyl-p-benzoquinone have been followed by reverse-phase chromatography. Chloride ions are formed from oxidation of chlorinated products. Generated carboxylic acids such as glycolic, glyoxylic, formic, malic, maleic, fumaric, and oxalic, have been detected by ion-exclusion chromatography. These products are destroyed by electro-Fenton, except oxalic acid that forms stable complexes with Fe3+. The efficient photodecarboxylation of such complexes by UV light explains the complete mineralization of MCPA by photoelectro-Fenton. (C) 2003 The Electrochemical Society.