Rationale: The Sm₂O₃-ZrO₂-HfO₂ system is a promising base for the development of a wide spectrum of new refractory materials. Reliable data on thermodynamic properties in this system are of significant importance for planning the preparation and application of high-temperature ceramics. Especially, they can be useful for calculation of the unknown phase equilibria in this system. Methods: The thermodynamic properties of the Sm₂O₃-ZrO₂-HfO₂ system were studied by the high-temperature mass spectrometric method. The samples in the system under consideration synthesized by the solid-state method were vaporized from a tungsten twin effusion cell using a MS-1301 magnetic sector mass spectrometer. Ionization of the vapor species effusing from the cell was carried out by electrons at an energy of 25 eV. Results: It was shown that, at temperatures below 2500 K, the main vapor species over the ceramics based on the Sm₂O₃-ZrO₂-HfO₂ system were SmO, Sm, and O corresponding to vapor composition over pure Sm₂O₃. The SmO, Sm, and O partial vapor pressures over the samples and the Sm₂O₃ activities were obtained in the temperature range 2319–2530 K. This allowed the excess Gibbs energy values to be determined. For comparison, the excess Gibbs energies in the Sm₂O₃-ZrO₂-HfO₂ system were also calculated by the semi-empirical Kohler, Toop, Redlich-Kister, and Wilson methods and optimized by the statistical thermodynamic Generalized Lattice Theory of Associated Solutions (GLTAS). Conclusions: The thermodynamic data calculated by the semi-empirical approaches at 2423 K were shown to be lower than the experimental values. However, the Toop and Wilson methods were found to be useful for evaluation of the excess Gibbs energy values at the Sm₂O₃ mole fraction less and higher than 0.32, respectively. The self-consistent thermodynamic description of the Sm₂O₃-ZrO₂-HfO₂ system was derived at high temperatures by optimization of the experimental results using the GLTAS.