The degradation of the methyl orange (MO) dye was investigated using advanced oxidation process (AOP) by exposing MO aqueous solution to UV irradiation after the addition of hydrogen peroxide (H2O2). The presence of H2O2 in MO solutions accelerated the dye degradation process. For instance, a complete degradation was achieved in 3 min when 0.1 mL of H2O2 aqueous solution was added to 20 mL of 0.078 mM MO solution compared to only 26% degradation achieved in 4 h in the absence of H2O2. The degradation of MO molecules was accompanied by the decolorization of the solution. A thorough investigation of the AOP process was carried out by studying the influence of the dye concentration, H2O2 dosage, UV irradiation power, and the area of the solution exposed to the UV source. It was found that, at the investigated ranges, the initial dye degradation rate increased with the increase in the initial dye concentration, UV irradiation power, and the surface area of the solution exposed to UV irradiation. An optimum concentration of H2O2 was found to exist for the MO degradation reaction. AOP by UV/H2O2 treatment was found to be a technically feasible approach for the decolorization of MO aqueous solutions. (C) 2010 Elsevier B.V. All rights reserved.