Ceramics based on the Sm₂O₃–Y₂O₃ and Sm₂O₃–HfO₂ systems is promising for development of advanced refractory materials including casting molds for gas turbine engine blades and thermal barrier coatings. Vaporization processes and thermodynamic properties of the Sm₂O₃–Y₂O₃ and Sm₂O₃–HfO₂ systems were studied by the Knudsen effusion mass spectrometric method. The samples used in the present study were synthesized by the solid-state method. Composition of the vapor over these systems at the temperature 2373 K was the same as over pure oxides. Temperature dependences of the partial pressures of the SmO and Sm vapor species were obtained over pure Sm₂O₃ in the temperature range 2265–2668 K and over Sm₂Hf₂O₇ in the temperature range 2222–2650 K. The partial pressures of the vapor species, the component activities, and the excess Gibbs energies were determined over a wide concentration ranges in the Sm₂O₃–Y₂O₃ and Sm₂O₃-HfO₂ systems at the temperature 2373 K. These thermodynamic functions exhibited negative deviations from the ideality and were compared with the data available for the La₂O₃–Y₂O₃, Gd₂O₃–Y₂O₃, ZrO₂–Y₂O₃ and La₂O₃–HfO₂, Gd₂O₃–HfO₂, Nd₂O₃–HfO₂ systems. Thermodynamic properties of these binary systems containing yttria and hafnia with rare earth oxides were discussed in the frame of the Generalized Lattice Theory of Associated Solutions. Correlations were found between the observed changes in the thermodynamic behavior of the systems mentioned and the relative number of bonds in the condensed phases when the second coordination sphere was taken into consideration.