The superfine lanthanum hafnate powder was synthesized by the glycol-citrate method. The phase transitions and the changes of microstructure of the La ₂Hf ₂O ₇ sample were studied under different conditions of thermal treatment (973–1673 K, 2–4 h). The vaporization processes of La ₂Hf ₂O ₇ were established at temperatures 2297–2500 K by the high temperature mass spectrometric method. The thermodynamic properties in the La ₂O ₃-HfO ₂ system were obtained at temperatures 2297 and 2445 K.