Abstract: High-temperature mass spectrometry was used for the first time to study vaporization processes and thermodynamic properties in the Fe2O3–ZrO2 and Fe2O3–HfO2 systems, and the results were compared with the corresponding data for the Fe2O3–La2O3 system. The vapor over these systems in the temperature range 1557– 1833 K was shown to consist predominantly of atomic iron and oxygen. From the determined partial pressures of Fe, the activities of FeO in the FeO–La2O3, FeO–ZrO2, and FeO–HfO2 systems at high temperatures were calculated and compared, demonstrating the validity of the acid–base concept for describing vaporization processes and thermodynamic properties of oxide systems.