In this study is demonstrated that hydrogen peroxide (H2O2) can be activated in the presence of a heterogeneous catalyst (Fe(3+-)containing ashes) to become a powerful oxidant. Fe3+ was immobilized by five different methods in fly ash using a thermal procedure. Leaching test carried out in 0.05 M Na2SO4 ( approximate to pH 2.8 adjusted with H2SO4) indicated that more of the 99wt.% of the iron ion stays in the solid catalysts and the presence of 11,02 does not alter this relation. It is concluded that radical species (HO2) are formed when H2O2 is activated by the immobilized Fe3+. This approach can be used in effluent treatment and the oxidation efficiency of reactive black 5 (RB5) was investigated in batch tests. RB5 (MW = 991 lambda(max) = 598 nm) is one of the most important azo dyes in the dyeing industry and the oxidation of 0.061 mM RB5 by heterogeneous hydrogen peroxide activation were therefore investigated in mild conditions. In theory, the complete oxidation of RB5 is a 152 electrons oxidation, if sulfur and nitrogen undergoes a complete oxidation, hence, the stoichiometric conversion of I mol of RB5 to CO2 requires up to 76 mol of hydrogen peroxide. Main experimental results show that it is possible to oxidize 0.061 mM RB5 (in a media 0.05 M Na2SO4, approximate to pH 2.8 adjusted with H2SO4) using a stoichiometric amount of H2O2. After 2h of treatment, reactive solutions were effectively colorless and 80% of the original chemical oxygen demand was removed. (c) 2007 Elsevier B.V. All rights reserved.