In this work, the synthesis. characterization and transformation of Fe3-x,MnxO4 (x = 0-0.53) spinels have been studied by Mossbauer spectroscopy, powder X-ray diffraction (XRD), thermal analyses (TG and DSC) and temperature programmed reduction (TPR) experiments. Mossbauer spectroscopy and XRD lattice parameters (a(o)) showed the presence of pure spinel phases with Mn incorporation mainly in the octahedral site replacing Fe2+. Upon thermal treatment in air, the spinels Fe3-xMnxO4 are oxidized to an Mn containing maghemite. At higher temperatures the maghemite is converted to the hexagonal alpha-Fe2O3 hematite phase. DSC analyses showed that the presence of manganese in the maghermite structure strongly decreases the transition temperature to hematite. The effect of these phase transformations on the catalytic carbon monoxide oxidation was investigated. It has been observed that the presence of Mn on the catalysts Fe3-xO4 does not significantly affect the catalytic activity at lower temperature. On the other hand, at temperatures higher than 200degreesC the magnetites are oxidized to maghemite/hematite and a great increase in the catalytic activity is observed. Catalytic experiments with the different iron oxides Fe3O4 (magnetite), gamma-Fe2O3 (maghernite) and alpha-Fe2O3 (hematite) showed that at lower temperatures, magnetite is the most active phase whereas at higher temperatures, hematite showed the highest activity. (C) 2003 Elsevier B.V. All rights reserved.