A Fenton-like system based on iron oxide surface modification by thermal treatment under H-2 flow was investigated. The materials were characterized by powder X-ray diffraction (XRD), chemical analyses, scanning electron microscopy (SEM), temperature programmed reduction (TPR) and Fe-57 Mossbauer spectroscopy. Results showed that the main iron oxides phases in the thermal treated material were goethite (alpha-FeOOH) and magnetite (Fe3O4). A decomposition study was carried out using a basic dye as a model compound (methylene blue). The reactions were monitored by electrospray ionization mass spectrometry (ESI-MS), revealing that the methylene blue dye was successively oxidized (hydroxylations) forming different intermediaries species. These results strongly suggest that highly reactive hydroxyl radicals generated from the reaction of H2O2 on the catalysts surface were responsible for the dye oxidation, proving that the material act as an efficient heterogeneous Fenton-like catalyst. Theoretical quantum mechanics calculations, DFT, were carried out in order to understand the degradation mechanism for methylene blue with goethite. (c) 2007 Elsevier B.V. All rights reserved.