Rationale: The Gd₂O₃-Y₂O₃ system possesses a number of practical applications, one of the most important of them being production of casting molds for gas turbine engine blades. The components of this system are often added to zirconia or hafnia to obtain high-temperature ceramics which are used for the development of thermal barrier coatings. However, Gd₂O₃ and Y₂O₃ are more volatile than zirconia or hafnia and may vaporize selectively during synthesis or usage of high-temperature materials which may lead to changes in their physicochemical properties. Therefore, information on the vaporization processes and thermodynamic properties of the Gd₂O₃-Y₂O₃ system is of great importance. Methods: High-temperature Knudsen effusion mass spectrometry was used to study the vaporization processes and to determine the thermodynamic properties of the Gd₂O₃-Y₂O₃ system. Measurements were performed with a MS-1301 mass spectrometer. Vaporization was carried out using a tungsten twin effusion cell containing the sample under study and pure Gd₂O₃ as a reference substance. Electron ionization at an energy of 25 eV was employed. Results: At the temperature of 2630 K, GdO, YO and O vapor species were identified over the samples in the Gd₂O₃-Y₂O₃ system. The Gd₂O₃ and Y₂O₃ activities and the vaporization rates of samples as functions of composition in the Gd₂O₃-Y₂O₃ system were derived from the partial pressures of the vapor species mentioned. Using these data the Gibbs energy of mixing and excess Gibbs energy of the hexagonal solid solution in this system were calculated at 2630 K. Conclusions: The thermodynamic properties of the Gd₂O₃-Y₂O₃ system, such as the activities of components and the excess Gibbs energy, obtained in the present study using Knudsen mass spectrometry at 2630 K, demonstrated significant negative deviations from ideal behavior. The vaporization rates of the samples were found to decrease as the Y₂O₃ content increased.