Rationale: The Y ₂O ₃-ZrO ₂-HfO ₂ system is a promising base for a wide range of high-temperature materials including ultra-high-temperature ceramics. At high temperatures of synthesis and application of these ceramics the components may vaporize selectively, leading to changes in chemical composition and exploitation properties of the materials. Therefore, study of the vaporization processes of ceramics based on the Y ₂O ₃-ZrO ₂-HfO ₂ system is of great importance. The thermodynamic properties of the Y ₂O ₃-ZrO ₂-HfO ₂ system obtained in the present study can be used for the prediction and modeling of the physicochemical properties of ultra-high-temperature ceramics. Methods: The present study was carried out by the high-temperature Knudsen effusion mass spectrometric method using the MS-1301 mass spectrometer, which was designed to study the physicochemical properties of non-volatile compounds. Vapor species effusing from the tungsten twin effusion cell, which was used for vaporization of the samples under study, were ionized by electron ionization with an ionization energy of 30 eV. Results: The gaseous phase over the samples of the Y ₂O ₃-ZrO ₂-HfO ₂ system was shown to consist of the YO, ZrO, ZrO ₂, HfO and O vapor species at a temperature of 2660 K. The YO, ZrO, ZrO ₂, HfO and O partial vapor pressures were obtained in a wide concentration range by the complete isothermal vaporization method, which allowed determination of the activities of Y ₂O ₃, ZrO ₂ and HfO ₂, Gibbs energies of mixing and excess Gibbs energies of the Y ₂O ₃-ZrO ₂-HfO ₂ system at 2660 K. Conclusions: Negative deviations from the ideal behavior were shown in the solid solutions of the Y ₂O ₃-ZrO ₂-HfO ₂ system at 2660 K. The excess Gibbs energies found in the present study were approximated using the Redlich-Kister representation. The possibility of application of the Kohler method to estimate the excess Gibbs energies of the Y ₂O ₃-ZrO ₂-HfO ₂ system was considered.