Thermodynamic properties and vaporization processes of the Gd ₂O ₃-HfO ₂ and Gd ₂O ₃-ZrO ₂ systems were studied by the Knudsen effusion mass spectrometric method (KEMS). The samples used in the present study were synthesized by sintering of pure oxides. The vapor species over the systems at the temperature 2600 K were GdO, ZrO, HfO, ZrO ₂, and atomic oxygen. The partial pressures of the vapor species, the component activities, and the excess Gibbs energies were obtained in the binary systems studied. It was illustrated that thermodynamic properties found in both systems had the negative deviations from ideality. The thermodynamic data found in the present study were also discussed using the results of modeling of the component activities based on the generalized lattice theory of associated solutions. Correlations were shown between the observed changes in the thermodynamic behavior of the Gd ₂O ₃-HfO ₂ and Gd ₂O ₃-ZrO ₂ systems and the relative number of bonds formed in the condensed phase when the second coordination sphere was taken into consideration.