The La ₂O ₃-HfO ₂ system is a base for many advanced materials used in high temperature technologies and nuclear industry. Information on vaporization processes and thermodynamic properties of ceramics based on the La ₂O ₃-HfO ₂ system is important to prevent the La ₂O ₃ selective vaporization during their synthesis or usage. The study was carried out by the Knudsen effusion mass spectrometric method. At 2337 K LaO and O were the main vapor species over the samples in the La ₂O ₃-HfO ₂ system. The dependence of the La ₂O ₃ activities on composition in the La ₂O ₃-HfO ₂ system was obtained by the differential mass spectrometric method. The HfO ₂ activities, excess Gibbs energies and Gibbs energies of mixing were evaluated in the solid solutions of the La ₂O ₃-HfO ₂ system at 2337 K using the Gibbs-Duhem equation and showed significant negative deviations from ideality. The solid solution presumably with the pyrochlore structure was characterized with the strongest negative deviations.