The thermodynamic properties and vaporization processes of the ceramics based on the La₂O₃-Y₂O₃-ZrO₂-HfO₂ and Sm₂O₃-Y₂O₃-ZrO₂-HfO₂ systems were studied using the Knudsen effusion mass spectrometric method (KEMS) at high temperatures. The ceramic samples in the systems mentioned were synthesized by the solid-state method. Vapor composition over the samples studied, the partial pressures of the vapor species, vaporization rates, and the lanthanoid oxide activities were determined in the temperature ranges 2339–2459 K and 2373–2461 K for the lanthana-containing and samaria-containing systems, respectively. The samples in the La₂O₃-Y₂O₃-ZrO₂-HfO₂ system were shown to be more volatile than those in the Sm₂O₃-Y₂O₃-ZrO₂-HfO₂ system. The data obtained evidenced negative deviations from the ideal behavior in the systems mentioned. Experimental thermodynamic data found at the temperature 2373 K were compared with the corresponding values calculated using the semi-empirical Kohler, Redlich-Kister, and Wilson approaches, as well as the Generalized Lattice Theory of Associated Solutions (GLTAS). It was shown that the data calculated by the semi-empirical methods were underestimated as compared to experimental values obtained using KEMS. It should be underlined that modeling based on GLTAS allowed more reliable data on the lanthanoid oxide activities in hafnia-based ceramics containing lanthana and samaria to be obtained in comparison with the semi-empirical methods mentioned.