The zirconium oxide based ceramics with addition of rare-earth metal oxides is widely used for production of high temperature materials. Knudsen effusion mass spectrometry was used to study vaporization processes and thermodynamic properties of solid solutions in the ZrO2-Lu2O3 system at the temperature 2700 K. The Lu, LuO, ZrO and ZrO2 were found as the main vapor species over the samples studied. The ZrO2 and Lu2O3 activities, the Gibbs free energy of formation, the excess Gibbs free energy of formation in the samples studied were calculated. The activities of lutetium oxide in all concentration range of condensed phase have low negative deviation from ideal case whereas zirconium dioxide activities have strong negative deviation from ideal case. © 2014 Elsevier Ltd. All rights reserved.