A solid state electrochemical cell featuring an yttria stabilized zirconia (YSZ) solid electrolyte was used to study the stability of M Ba-ferrnite (BaFe12O19). The oxygen coulometric titration curves obtained in the temperature range between 700 and 1000 degrees C all showed well-defined plateaus with stable open circuit voltage values. The plateaus were indicative of an invariant solid state decomposition reaction of BaFe12O19 into BaFe2O4 and Fe3O4. The thermodynamic stability regime was obtained from the temperature dependence of the plateau potential. At 1000 degrees C, the lower stability limit for BaFe12O19 was found to be 3.2 X 10(-6) atm oxygen while this limit decreased drastically down to 3.4 X 10(-7) atm at 700 degrees C.