Ni-Zn ferrite is a magnetic material that is used commercially in high-power devices, such as transformer and inductor cores. The important properties of this material are highly dependent on its oxygen non-stoichiometry. The processing of ferrites having the desired oxygen non-stoichiometry requires knowledge of the relationship between non-stoichiometry and the temperature and the partial pressure of oxygen.Two Ni-Zn ferrites having compositions in the technologically important range were prepared by a spray-drying technique. The initial degree of oxidation was determined by coulometric titration. The PO2 vs, temperature phase diagrams for these ferrites were then determined by thermogravimetry under atmospheres of varying PO2, 2x10(-4)-1 atm, in the 800-1400 degrees C range. Isostoichiometric lines are shown on plots of log PO2, vs. 1/T. The heat of oxidation was calculated from the slopes of the isostoichiometric lines.