Rationale: Materials based on the La ₂O ₃-Y ₂O ₃-HfO ₂ system are promising for the production of highly refractory ceramics, e.g., thermal barrier coatings and molds for casting of elements of gas turbine engines. When these ceramics are synthesized or used at high temperatures, selective vaporization of components may take place, resulting in changes in the physicochemical properties of the materials. Consequently, development of materials based on the La ₂O ₃-Y ₂O ₃-HfO ₂ system requires information on vaporization in this system as well as on its thermodynamics, without which prediction and modeling of their physicochemical properties are impossible. Methods: Vaporization processes and thermodynamic properties in the La ₂O ₃-Y ₂O ₃-HfO ₂ system were studied by the high-temperature Knudsen effusion mass spectrometric method using a MS-1301 mass spectrometer. Electron ionization of vapor species was employed at an ionization energy of 25 eV. The samples under study and reference substances were vaporized from a tungsten twin effusion cell. Results: At 2337 K the main vapor species over samples in the La ₂O ₃-Y ₂O ₃-HfO ₂ system were shown to be LaO, YO and O. The partial pressures of the vapor species mentioned and the La ₂O ₃ and Y ₂O ₃ activities in the samples were obtained at 2337 K. The Gibbs energies of mixing and excess Gibbs energies were found in the solid solution of this system. Conclusions: Vaporization of ceramics based on the La ₂O ₃-Y ₂O ₃-HfO ₂ system at 2337 K led to selective transition of La ₂O ₃ and Y ₂O ₃ to the gaseous phase, with the La ₂O ₃ vaporization rate being higher than that of Y ₂O ₃. The directions of composition changes of samples due to their vaporization at 2337 K were determined. In the solid solution of this system negative deviations from ideal behavior were found. The ability to estimate the excess Gibbs energies in the solid solution of the La ₂O ₃-Y ₂O ₃-HfO ₂ system by the Kohler method was shown.