The oxidation of hausmannite, Mn3O4, of different particle sizes has been studied by thermogravimetry with a large range of linear heating programs under an oxygen pressure of 10(5) Pa. For high heating rate (600 degreesC/h) and whatever the particle size, Mn3O4 was converted to alpha -Mn2O3. For small particle size (32 nm) and low heating rate (3 degreesC/h) the oxidation shows the formation of two metastable manganese oxides, a cation deficient spinel of the formula Mn2O2.92 with Mn2+ ions oxidation and Mn5O8 associated to Mn3+ ions oxidation. The difference in the kinetic behavior of Mn2+ and Mn3+ ions is interpreted with regard to a structural change from the cation deficient spinel to Mn5O8 initiated by the oxidation temperature.