The decolorization of azo dye Orange II using a montmorillonite/H2O2 system and a heterogeneous Fenton system, with montmorillonite KSF as a catalyst, has been studied. A series of experiments were performed to analyze the effects of several variables, including pH, KSF dosage, H2O2 dosage and the initial Orange II concentration. The results revealed that under proper conditions, relatively high decolorization efficiency (more than 90%) could be achieved in only 40 min when the initial concentration of Orange II was 20 mg/L Response surface methodology (RSM) was applied with a Box-Behnken design (BBD) of a series of experiments, which showed that, of the range of variables studied, the order of influence is pH>KSF dosage > H2O2 dosage. A mathematical model was established and the optimal conditions were determined by RSM. Sequential experiments showed that KSF, re-used from a former reaction, performed well for 4 further runs. Comparisons between homogeneous and heterogeneous catalytic oxidation showed that unreleased free ferrous iron and structural iron in the KSF particles were responsible for the catalysis of the oxidation of Orange II in the montmorillonite/H2O2 system at pH 4. However, at pH 3, the predominant catalytic iron species was the released free ferrous iron. (C) 2011 Elsevier By. All rights reserved.