Abstract: The processes of the TiO₂–CeO₂–ZrO₂ system evaporation were studied by high-temperature mass spectrometry. The values of the activities of TiO₂ and CeO₂ at 2250 K indicate a positive deviation TiO₂ activity from the ideal behavior and result an increase in the activity coefficient of CeO₂ compared to the binary system CeO₂–ZrO₂.