The equilibrium phase diagram was investigated for hafnium-hydrogen binary system. Using a modified Sieverts apparatus, pressure-composition-temperature (P-C-T) relationships were measured for the hafnium-hydrogen systems at low temperature. Because the allotropic transformation temperature of pure hafnium is about 2000 K, the hafnium-hydrogen phase diagram could not be obtained directly at high temperature. Therefore, based on the Duhem-Margules equation, the partial molar free energy of hydrogen can be estimated at high temperature.