Electrical resistivity measurements to pressures of 83 GPa and temperatures ranging from 300 K to 4300 K confirm the presence of both crystalline and liquid metallic phases of FeO at pressures above 60--70 GPa and temperatures above 1000 K. By experimentally determining the melting temperature of FeO to 100 GPa and of a model core composition at 83 GPa, we find that the solid-melt equilibria can be described by complete solid solution across the Fe-FeO system at pressures above 70 GPa. Our results indicate that oxygen is a viable and likely candidate for the major light alloying element of the Earth's liquid outer core. The data suggest that the temperature at the core-mantle boundary is close to 4800 (¿500) K and that heat lost out of the core accounts for more than 20% of the heat flux observed at the surface. ¿ American Geophysical Union 1991