Rationale: Systems containing zirconia, hafnia, and rare earth oxides are indispensable in various areas of high-temperature technologies as a basis of ultra-high refractory ceramics. Exposure of these materials to high temperatures may result in unexpected selective vaporization of components or phase transitions in the condensed phase leading to changes in physicochemical properties. Consequently, reliable application of the ceramics based on systems such as Sm ₂O ₃-ZrO ₂-HfO ₂ is impossible without data on its vaporization processes and thermodynamic properties, which may be used to predict the physicochemical characteristics of the ultra-high refractory ceramics. Methods: Ceramics based on the Sm ₂O ₃-ZrO ₂-HfO ₂ system were obtained by solid-state synthesis and characterized by X-ray fluorescence and X-ray phase analyses. The vaporization and thermodynamics of the system considered were examined by the high-temperature mass spectrometric method using a MS-1301 magnetic sector mass spectrometer with a tungsten twin effusion cell. Vapor species effusing from the cell were ionized by electrons with an energy of 25 eV. Results: The main vapor species over the Sm ₂O ₃-ZrO ₂-HfO ₂ system were shown to be SmO, Sm, and O at a temperature of 2373 K, indicating selective vaporization of Sm ₂O ₃ from the samples. The partial pressures of these vapor species and the Sm ₂O ₃ activities were determined in the Sm ₂O ₃-ZrO ₂-HfO ₂ system and allowed the excess Gibbs energies to be evaluated. These excess Gibbs energy values were compared with the results obtained by the semi-empirical and statistical thermodynamic approaches. Conclusions: The data obtained in this study showed negative deviations from the ideal behavior in the Sm ₂O ₃-ZrO ₂-HfO ₂ system at 2373 K. The results calculated according to the semi-empirical methods and statistical thermodynamic Generalized Lattice Theory of Associated Solutions were in agreement with each other. Thus, this evidenced the desirability of further experimental investigation of the Sm ₂O ₃-ZrO ₂-HfO ₂ system by the high-temperature mass spectrometric method.