BACKGROUND: This study compared the decolorization behaviors of a model azo dye of methyl red in Na2SO4 and NaCl media by electro-Fenton process, in which FeSO4 was added into the solution, while H2O2 was efficiently generated on a graphite-polytetrafluoroethylene cathode. Parameters such as PH, cathodic potential, electrolyte concentration, and initial dye concentration as well as the treatment time were investigated to disclose different decolorization behaviour. RESULTS: Decolorization in NaCl medium had an advantage over that in Na2SO4 medium not only in performance but also in its suitability for application, which seemed less sensitive to variations in parameters and thus broadened the optimal performance range. In NaCl medium, indirect oxidation by active chlorine contributed greatly to the decolorization, and the removal of methyl red obeyed pseudo-first-order kinetics. CONCLUSION: Electro-Fenton oxidation in the present system in the presence of NaCl would is a promising procedure for azo dye waste-water treatment. (C) 2009 Society of Chemical Industry