Heterogeneous catalysts based on magnetic mixed iron oxides (MO center dot Fe2O3; M: Fe, Co, Cu, Mn) were used for the decolorization of several synthetic dyes (Bromophenol Blue, Chicago Sky Blue, Cu Phthalocyanine, Eosin Yellowish, Evans Blue, Naphthol Blue Black, Phenol Red, Poly B-411, and Reactive Orange 16). All the catalysts decomposed H2O2 yielding highly reactive hydroxyl radicals, and were able to decolorize the synthetic dyes. The most effective catalyst FeO-Fe2O3 (25 mg mL(-1) with 100 mmol L-1 H2O2) produced more than 90% decolorization of 50 mg L-1 Bromophenol Blue, Chicago Sky Blue, Evans Blue and Naphthol Blue Black within 24 h. The fastest decomposition proceeded during the first hour of the reaction. In addition to dye decolorization, all the catalysts also caused a significant decrease of chemical oxygen demand (COD). Individual catalysts were active in the pH range 2-10 depending on their structure and were able to perform sequential catalytic cycles with low metal leaching. (c) 2006 Elsevier B.V. All rights reserved.