Nb-substituted goethites have been prepared and characterized by Mossbauer spectroscopy, XRD, FTIR and BET surface area measurements. The doublet formation in Mossbauer spectra and the decreasing of the crystallinity shown in XRD analyses indicated that the Fe domain size is small, which may be the result of either Fe3+ substitution for Nb5+ in the goethite structure or simply the formation of small particle-size goethite when Nb is present. FTIR analyses showed shifts and broadening of the bands as result of the incorporation of Nb5+ ions into the alpha-FeOOH structure. The insertion of Nb in the goethite structure caused a significant increase in the BET surface area of the material. The prepared materials were investigated for the H2O2 decomposition and the Fenton reaction in the oxidation of methylene blue dye. It wits observed that the introduction of Nb during the synthesis of goethite produced a strong increase in the activity for the dye contaminant oxidation by H2O2. Theoretical quantum DFT calculations were carried out in order to understand the degradation mechanism for methylene blue with goethites. (c) 2008 Elsevier B.V. All rights reserved.