The ceramics based on the Y ₂ O ₃ -ZrO ₂ system is promising for development of refractory materials including ultra-high temperature ceramics. In the present study the samples of the Y ₂ O ₃ -ZrO ₂ system were synthesized by the solid-state method. The vaporization processes and thermodynamic properties of the Y ₂ O ₃ -ZrO ₂ system were studied by the Knudsen effusion mass spectrometric method. The vapor composition, the YO, ZrO, ZrO ₂ and O partial pressures over the samples as well as the Y ₂ O ₃ and ZrO ₂ activities of the Y ₂ O ₃ -ZrO ₂ system were determined at the temperature 2660 K. Thermodynamic properties (i.e. excess Gibbs energies) of the Y ₂ O ₃ -ZrO ₂ system obtained in the present study can be described in terms of the subregular solution model.