The intrinsic oxygen fugacity of an individual iron-bearing mineral is a theoretically definable quantity. It can be determined experimentally if the sensitivity of the method is high enough compared to the steepness of the slope of the free energy curve against the composition of the mineral phase. In order to achieve such a delicate measurement, a solid electrolyte method that incorporates the f O2 negative feedback principle has been developed and applied to several iron-bearing minerals under low total pressure. The results appear to promise usefulness of this method for solving various mineralogical and petrological problems which involve iron-bearing oxides and silicates at temperatures above about 450°C.