To promote treatment efficiency, an efficient undivided electrochemical system using a graphite-polytetrafluoroethylene (PTFE) cathode with high production rate of hydrogen peroxide and current efficiency under wider pH ranges was applied to the electro-Fenton oxidation of a model azo dye, methyl red. The influences of operating parameters including electrolyte Na2SO4 concentration, cathodic potential, pH, Fe2+ concentration and initial methyl red concentration were investigated. At the conditions of cathodic potential of -0.55 V versus SCE, pH 3, Fe2+ concentration of 0.2 M, methyl red of initial concentration of 100 mg/L could be removed 80% in 20 min. Due to the consumption of ferrous ion and formation of hard-to-treat intermediates during treatment, the removal of methyl red was found degraded at two different stages, and the second stage was much slower than the first one. (C) 2007 Elsevier B.V. All rights reserved.