The phase stability and oxygen stoichiometry of manganese ferrite were investigated for dependence on the oxygen partial pressure at 400 degrees C-900 degrees C. As starting material, a reactive Mn-Fe-hydroxide precursor with defined composition was used. The samples were characterized by X-ray diffractometry, Mossbauer spectroscopy and magnetic measurements. The oxygen non-stoichiometry delta in differently prepared samples MnFe2O4+delta was determined by a solid electrolyte-based coulometric technique (SEC).