The Hf-Ge system has been studied via experiment, first-principles calculations and thermodynamic modeling. 15 annealed Hf-Ge alloys were analyzed by means of X-ray diffraction, scanning electron microscope with energy dispersive X-ray analysis, differential thermal analysis and high temperature reaction calorimetry. The major experimental results are as follows: (I) the heat contents of the Hf3Ge and HfGe2 phases were measured from 400 to 1050 A degrees C; (II) the enthalpy of formation at 298 K for the HfGe2 phase (-67 +/- A 4 kJ/mol atoms) was measured; (III) the invariant reaction temperature of Liquid a dagger" (Ge) + HfGe2 was determined to be at 932.6 +/- A 2.3 A degrees C; and (IV) the previously reported phases HfGe and Hf3Ge2 were not found in the present experiment. In addition, the enthalpies of formation at 0 K for the Hf3Ge, Hf2Ge, Hf5Ge3, Hf5Ge4, Hf3Ge2, and HfGe2 phases were computed from first-principles calculations in order to provide the necessary energy levels for the thermodynamic properties of the binary compounds. Based on the present experimental results and first-principles calculations as well as the reliable literature data, the Hf-Ge system was modeled by using a CALPHAD approach.