Rationale: Materials based on the Gd₂O₃-ZrO₂-HfO₂ system are promising for a wide range of high-temperature technological applications, such as for obtaining thermal barrier coatings in the aviation and space industry, as well as advanced materials in nuclear power applications. Experimental studies of the ceramics based on this system by the Knudsen effusion mass spectrometric method provides such valuable information as the vapor composition over the samples and enables derivation of the thermodynamic functions. Methods: Samples of ceramics in the Gd₂O₃-ZrO₂-HfO₂ system were synthesized and analyzed by X-ray fluorescence and diffraction techniques. The vaporization processes and partial pressures of the vapor species over the samples were obtained by the high-temperature mass spectrometric method using the ion current comparison method. The derived thermodynamic functions were optimized within the Generalized Lattice Theory of Associated Solutions (GLTAS) approach. Results: At the temperature of 2600 K, the GdO, ZrO, ZrO₂, HfO, and O vapor species were found over the samples, but only for the GdO and ZrO species were accurate experimental data on the partial pressures obtained. In all the ceramic samples, the Gd₂O₃ activity was determined. On this experimental basis, modeling within the GLTAS approach was performed. It allowed the evaluation of the Gd₂O₃, ZrO₂, and HfO₂ activities in the system under study and a rough approximation of the excess Gibbs energy as a function of composition to be obtained. Conclusions: At 2600 K the Gd₂O₃-ZrO₂-HfO₂ system is characterized by negative deviations of its thermodynamic properties from the ideal behavior. Consistency of the obtained modeling results indicate reasonable uniformity of the energy parameters of the lattice model derived in calculations of the hafnia-containing oxide systems, which may be used in further modeling of multicomponent systems.