The oxidative degradation of insecticide metomyl as active ingredient and of its commercial formulation Lanate 20 L by an electro-Fenton process using a carbon felt cathode and iron ions as catalyst was compared with different transition metal ions such as, cobalt, silver, and copper as catalyst to produce hydroxyl radicals. The effect of catalyst nature and concentration on the degradation kinetics and COD removal was investigated and optimized. For all metal ions under study as catalyst, there is an optimal concentration value for degradation kinetics and solution COD removal efficiency. Fe(III) was confirmed to be the most efficient catalyst among the metals ions tested. The metomyl oxidative degradation was found to obey apparent first-order reaction kinetics, and the absolute rate constant of the oxidation between metomyl and hydroxyl radicals at pH 3.0 was obtained by competitive kinetics method, being 5.42 x 10(9) L mol(-1) s(-1).