Due to the very high dispersion of mixed-valence spinel ferrites prepared by 'soft chemistry' it becomes possible to oxidize in the spinel lattice, not only ferrous ions but also different transition metal ions (Cr3+, Mn2+, Mn3+, Mo3+, Mo4+, V2+, V3+, Cu+), by 'soft' oxidation between 150 and 500 degrees C, under formation of non-stoichiometric spinels cation vacancies. The oxidation state with the coordination and the oxidation temperature of the cations have been determined in copper-manganese ferrites by derivative thermogravimetry (DTG), based on the specific solid-state reactivity of cations in the redox reaction. For Go-modified non-stoichiometric spinels, which present interesting magneto-optical properties due to their exceptional high coercivity (1000-3500 Oe) and whose cation distribution has been determined by the method described above, close correlations have been found between the oxidation state of the spinel and the variation of the coercivity.