Abstract: The thermodynamic properties of the Sm₂O₃–Y₂O₃–HfO₂ system at 2500 K were modeled based on the generalized lattice theory of associated solutions using experimental data obtained by high-temperature mass spectrometry. A comparison was made between the results of the calculations of the thermodynamic properties of the Sm₂O₃–Y₂O₃–HfO₂ system using two independent approaches to modeling based on this theory: from the experimental data on the properties of the ternary system and from the data on its boundary binary systems Sm₂O₃–Y₂O₃, Sm₂O₃–HfO₂, and Y₂O₃–HfO₂. It was shown that the chemical potentials of the components that were calculated in the studied ternary system using the above approaches much better fit the experimental data in the former case.