The electro-Fenton process was used to assess the decoloration of Orange II in aqueous solutions. Hydrogen peroxide (H2O2) was electro-generated by reduction of dissolved oxygen (DO) in acidic solutions containing dilute supporting electrolyte. This oxidation process allows the production of active intermediates, which react on the organic compounds, leading to their mineralization. Decoloration experiments were performed in the presence of perchlorate, chloride and sulfate electrolyte media at pH 3.0 under potential controlled electrolysis conditions. Effect of operational parameters such as cathodic potential, oxygen mass flow rate and inert supporting electrolyte type and concentration were investigated to find the best experimental conditions for achieving overall dye decoloration. Complete decoloration of Orange II occurs in less than 90 min. In addition, effect of cathode surface area and electrolyte concentration in the accumulation of H2O2 and behavior of the Fe3+/Fe2+ in the electro-Fenton system was examined. Results show that potential of -0.5 V vs. SCE, 0.05 M NaClO4 electrolyte, O-2 sparging rate of 8 ml/min and diluted dye concentration were the best conditions for decoloration of the Orange II by electro-Fenton process. The mineralization of the initial pollutant was investigated by total organic carbon measurements that show a 75% mineralization of 2 x 10(-5) M dye at 180 min. Also, degradation by-products were identified with GC-MS analysis. (C) 2007 Elsevier B.V. All rights reserved.