Hausmannite, Mn3O4, with a specific surface area of 10 m(2)/g has been oxidized at low temperatures (300 degrees C < T < 425 degrees C). In this temperature range both Mn2+ and Mn3+ cations are oxidized. The oxidation of Mn2+ to Mn3+ is governed by a diffusion process and the spinel structure is stable up to 425 degrees C. Oxidation of Mn3+ to Mn4+ leads to the crystallization of the Mn5O8 phase. Careful oxidation studies show the formation of two metastable manganese oxides, Mn5O8 and a defect spinel of the formula Mn2O2.78. The coordination numbers and the cation-anion distances in Mn3O4, gamma-Mn2O2.78 and alpha-Mn2O3 obtained by EXAFS (extended X-ray absorption fine structure) are compared.