Thermodynamic properties of the Gd ₂ O ₃ -Y ₂ O ₃ -HfO ₂ and La ₂ O ₃ -Y ₂ O ₃ -HfO ₂ systems were obtained by Knudsen effusion mass spectrometric method at the temperatures 2500 K and 2337 K, respectively. Description of the thermodynamic properties of these systems was obtained using the Barker theory. It was illustrated that negative deviations from the ideality in these systems were related to the formation of the Me-O-Y and Y-O-Hf bonds in the solid solutions. Accuracy of the optimization of the thermodynamic data in the Gd ₂ O ₃ -Y ₂ O ₃ -HfO ₂ system using the Barker theory was checked in comparison with the traditional Redlich-Kister approximation approach.