New results from high-pressure multi-anvil and diamond anvil cell experiments show that the Fe3+ content of (Mg, Fe)O is low at high pressures and temperatures, and relatively independent of oxygen fugacity. At 18 GPa and 1000 ¿C, the maximum solubility of Fe3+ in Mg0.8Fe0.2O ranges from approximately 2% Fe3+/&Sgr;Fe at the Fe-FeO buffer to 3--7% Fe3+/&Sgr;Fe at the Re-ReO2 buffer. These low values are likely due to a high-pressure phase transition in the system Fe3O4-MgFe2O4, resulting in preferential partitioning of Fe3+ into the high-pressure phase. (Fe, Mg)Fe2O4 would be exsolved in (Mg, Fe)O depending on the oxygen fugacity, which could have significant effects on transport properties. Exsolved (Fe, Mg)Fe2O4 could also be useful as an oxygen barometer. ¿ 1998 American Geophysical Union