Iron-nickel mixed oxides containing up to 50 mol% of NiO were prepared by firing the corresponding co-precipitated hydrous oxides; characterization was performed by X-ray diffraction, infrared spectroscopy, magnetic susceptibility, electrical conductivity and thermoelectric power measurements. A non-stoichiometric ferrite phase was formed when a sample containing 20 mol% NiO was sintered at 1050 degrees C. This phase had two- to three-fold higher conductivity than either Fe2O3 or the stoichiometric ferrite (NiFe2O4). The thermoelectric power of this phase indicated a sharp change of charge carriers from n- to p-type near 350 degrees C. This non-stoichiometric ferrite phase was stable only in a small temperature range and dissociated into alpha-Fe2O3 and stoichiometric ferrite above 1200 degrees C. Samples containing 5 and 10 mol% NiO also had small fractions of this non-stoichiometric ferrite phase when sintered at 1050 degrees C.