The electro-Fenton treatment of sulfachloropyridazine (SCP), a model for sulfonamide antibiotics that are widespread in waters, was performed using cells with a carbon-felt cathode and Pt or boron-doped diamond (BDD) anode, aiming to present an integral assessment of the kinetics, electrodegradation byproducts, and toxicity evolution. H2O2 electrogeneration in the presence of Fe2+ yielded (OH)-O-center dot in the solution bulk, which acted concomitantly with (OH)-O-center dot adsorbed at the anode (BDD((OH)-O-center dot)) to promote the oxidative degradation of SCP (k(abs,SCP) = (1.58 +/- 0.02) x 10(9) M-1 s(-1)) and its byproducts. A detailed scheme for the complete mineralization was elucidated. On the basis of the action of (OH)-O-center dot onto four different SCP sites, the pathways leading to total decontamination includes fifteen cyclic byproducts identified by HPLC and GC-MS, five aliphatic carboxylic acids, and a mixture of Cl-, SO42-, NH4+, and NO3- that accounted for 90-100% of initial Cl, S, and N. The time course of byproducts was satisfactorily correlated with the toxicity profiles determined of Vibrio fischeri luminescence. 3-Amino-6-chloropyridazine and p-benzoquinone were responsible for the increased toxicity during the first stages. Independent electrolyses revealed that their toxicity trends were close to those of SCP. The formation of the carboxylic acids involved a sharp toxicity decrease, thus ensuring overall detoxification.