The decolourisation of an azo dye, Methyl Orange, by an electrochemical technology was studied in a reaction cell with a working volume of 1 dm(3), using graphite for both electrodes (anode and cathode). No decolourisation was detected in the treatment of pure solutions of Methyl Orange, but significant decolourisation was observed in the presence of NaCl, the influence of which was established. The extent of decolourisation, treatment time, and electrical consumption largely depended on the applied potential difference; the optimum value being around 5V. To achieve effective decolourisation, the NaCl concentration should be increased as the concentration of azo dye increases. Thus, almost complete decolourisation was achieved in 60 min for an effluent containing 40 kg m(-3) NaCl and go g m(-3) of dye. Increasing temperature (from 20 to 40 degreesC) did not improve the efficiency of electrochemical oxidation. (C) 2004 Society of Chemical Industry.