The vaporization processes and thermodynamic properties of twenty samples in the Gd ₂ O ₃ -Y ₂ O ₃ -HfO ₂ system in a wide concentration range were studied by high-temperature Knudsen effusion mass spectrometry. At the temperature 2500 K the main vapor species were GdO, YO, and O. From the measured values of their partial pressures activities of the Gd ₂ O ₃ and Y ₂ O ₃ as functions of concentration were determined. The HfO ₂ activity was calculated by the Gibbs-Duhem integration. The resulting excess Gibbs energy function exhibited negative deviations from ideal behavior. For the optimization of the obtained thermodynamic functions of the ternary Gd ₂ O ₃ -Y ₂ O ₃ -HfO ₂ system the generalized Barker-Guggenheim theory of associated solutions was applied. In an alternative computing procedure the same model was used, but the optimization was based on the available literature data for the corresponding binary Gd ₂ O ₃ -Y ₂ O ₃ , Gd ₂ O ₃ -HfO ₂ , and Y ₂ O ₃ -HfO ₂ systems. Thus, the Barker lattice model may be used for estimation of the thermodynamic functions of a ternary system on the basis of the data for the corresponding binary sub-systems.