High-pressure/temperature phase relationships in Fe3O4 have been determined up to 34 GPa and 600 ¿C using an energy dispersive X ray diffraction method on the Cornell High Energy Synchrotron Source. This experiment has shown that the slope (dT/dP) of the transformation from magnetite to its high-pressure phase is -45 ¿C/GPa with increasing pressure. The average slope of the reversion boundary from high-pressure phase to magnetite is 170 ¿C/GPa. Neither boundary is considered to represent the true Clapeyron boundary. The Clapeyron boundary for the transition from magnetite to high-pressure magnetite is tentatively determined as -68 ¿C/GPa when relevant effects are considered. These results are then used to calculate some of the phase relations in the FeO phase diagram. These preliminary studies show that the rapid data acquisition with synchrotron radiation will make it possible in future investigations to measure rates of reaction and to correct for the effects of sluggishness.