The degrdn. of herbicides 4-chlorophenoxyacetic acid (4-CPA), 4-chloro-2-methylphenoxyacetic acid (MCPA), 2,4-dichlorophenoxyacetic acid (2,4-D) and 2,4,5-trichlorophenoxyacetic acid (2,4,5-T) in aq. medium of pH 3.0 was comparatively studied by anodic oxidn. and electro-Fenton using a B-doped diamond (BDD) anode. All solns. are totally mineralized by electro-Fenton, even at low current, being the process more efficient with 1 mM Fe2+ as catalyst. This is due to the prodn. of large amts. of oxidant hydroxyl radical (OH.bul.) on the BDD surface by H2O oxidn. and from Fenton's reaction between added Fe2+ and H2O2 electrogenerated at the O2-diffusion cathode. The herbicide solns. are also completely depolluted by anodic oxidn. Although a quicker degrdn. is found at the 1st stages of electro-Fenton, similar times are required for achieving overall mineralization in both methods. The decay kinetics of all herbicides always follows a pseudo 1st-order reaction. Reversed-phase chromatog. allows detecting 4-chlorophenol, 4-chloro-o-cresol, 2,4-dichlorophenol and 2,4,5-trichlorophenol as primary arom. intermediates of 4-CPA, MCPA, 2,4-D and 2,4,5-T, resp. Dechlorination of these products gives Cl-, which is slowly oxidized on BDD. Ion-exclusion chromatog. reveals persistent oxalic acid in electro-Fenton by formation of Fe3+-oxalato complexes, which are slowly destroyed by OH.bul. adsorbed on BDD. In anodic oxidn., oxalic acid is mineralized practically at the same rate as generated. [on SciFinder (R)]