Solutions of 2.5L with 209.3mgL −1 of the azo dye Carmoisine in 0.050M SO 4 2− , ClO 4 − or Cl − have been comparatively treated by electrochemical oxidation with electrogenerated H 2 O 2 (EO-H 2 O 2 ) and electro-Fenton (EF) with 0.5mM Fe 2+ as catalyst at constant current density. Experiments were made using a recirculation flow plant containing a reactor with a boron-doped diamond (BDD) anode and an air-diffusion cathode to allow H 2 O 2 generation. The dye and its oxidation products were oxidized by hydroxyl radical and/or HClO formed at the anode from water or Cl − oxidation, respectively, in EO-H 2 O 2 , as well as by hydroxyl radical produced in the bulk from Fenton’s reaction between added Fe 2+ and generated H 2 O 2 in EF. In both methods, the decolorization process was always much faster in Cl − medium because of the quick oxidation of colored compounds by HClO, being enhanced by increasing current density and Cl − concentration. The solutions with SO 4 2− or ClO 4 − were more rapidly decolorized in EF due to the higher oxidation power of hydroxyl radicals in the bulk. Regarding the overall decontamination, a poor and similar mineralization of about 50% was obtained by EO-H 2 O 2 at 480min in all the supporting electrolytes at 100mAcm −2 . The comparative EF treatments were always much more powerful, being SO 4 2− the most favorable medium leading to 76% mineralization with the lowest energy consumption. Up to 15 aromatic products were detected by GC–MS and short-linear carboxylic acids like tartronic, oxalic, oxamic and formic were quantified by ion-exclusion HPLC. The large persistence of Fe(III)-oxalate complexes accounted for the partial mineralization of the Carmoisine solution in EF. Nitrate and sulfate were the major ions released during the mineralization process.