A study involving equilibration at 1573°K of fused uranium tetrafluoride and molten irradiated uranium has permitted calculation of the free energy of formation of plutonium trifluoride at 1573°K. During equilibration it is assumed that plutonium is extracted into molten uranium fluoride as represented by one of the following equations: Free energy values for the formation of plutonium trifluoride are calculated from experimentally determined equilibrium constants and the free energy of formation of uranium tetrafluoride, using the expression: The free energy of formation at 1573°K for PuF3 was found to be 93 ° 1.5 kcal/equivalent as compared to an estimated value of 94 kcal/equivalent at 1500°K based on earlier work by Brewer, Bromley, Gilles, and Lofgren.