A series of Cr-containing magnetites, Fe3-xCrxO4 (x = 0-00, 0.07, 0.26, 0.42 and 0.51) has been prepared using conventional co-precipitation method. Mossbauer and powder X-ray diffraction measurements suggested the formation of the spinel crystalline phase with initial substitution of Fe-oct(3+) by Cr3+ and for higher Cr contents, chromium also replaces Fe-oct(2+) and Fe-tet(3+) in the crystalline structure. N-2 adsorption/desorption revealed that the presence of Cr has a remarkable effect on the texturial properties of the material decreasing the pore diameter from meso- to micropore with a significant increase on the BET surface area. Thermal analyses (TG and DTA) coupled with XRD and Mossbauer showed that thermal treatment up to 270 degrees C with O-2 leads to the oxidation of Fe-oct(2+) producing Cr-substituted maghemite but at 600 degrees C the cubic maghemite is converted to the hexagonal hematite and Cr is expelled from the iron oxide structure. The reactivity of Fe3-xCrxO4 was investigated using two H2O2 reactions, i.e. the decomposition to O-2 and the oxidation of the dye methylene blue used as model contaminant. The obtained results showed that the presence of Cr directly promoted the H2O2 decomposition. On the other hand, the presence of small concentration of Cr, i.e. Cr-0.07 and Cr-0.21, caused a significant increase on the activity for the oxidation of the dyes with complete discoloration and high degree of mineralization. The higher activity of Fe2.93Cr0.07O4 was discussed in terms of a coupling of the redox pairs Fe3+/Fe2+ and Cr2+/Cr3+ producing a more efficient regeneration of the Fenton active specie Fe2+. (C) 2007 Elsevier B.V. All rights reserved.