The experimental design methodology was applied having in mind the optimization of the azo dye Orange II degradation using the Fenton's reagent (mixture of H2O2 and Fe2+). The variables considered were the temperature, H2O2 concentration and Fe2+:H2O2 ratio, for a dye concentration of 0.3 mM and pH 3. The multivariate experimental design allowed to develop quadratic models for: (i) color removal and (ii) total organic carbon (TOC) reduction (both after 2 h of reaction), which were adequate to predict responses in all the range of experimental conditions used. Under the optimum conditions, performances of 99.7 and 70.7% for color and TOC removal, respectively, were experimentally reached. It was found that both H2O2 concentration and temperature have an important effect in the organic matter degradation efficiency. (c) 2005 Elsevier B.V. All rights reserved.