New manganese defect metastable ferrites with spinal structure were prepared by oxidation at a temperature lower than 500°C of submicron manganese substituted magnetites, Fe3-xMNxO4( ). X-ray diffraction measurements and thermal analysis studies have been carried out on these systems. Low-temperature magnetic measurements have also been carried out. With these studies, it has been possible to depict the cation distribution in substituted magnetites and oxidized compounds, and to understand the complex process of oxidation. In defect spinels, vacancies are formed not only from oxidation of Fe2+ions, but also of Mn2+and Mn3+ions. Manganese-rich compound with x = 0.97 having high content of Mn3+and Mn4+ions show an important increase in coercive force, which exceeds 700 Oe at 4.2 K.