The thermodynamic properties of the Gd O -Y O -HfO system, i.e., the component activities in the samples and the excess Gibbs energy, obtained in the present study at 2500 K, exhibited negative deviations from ideal behavior. The concentration dependence of excess Gibbs energy of the Gd O -Y O -HfO system was approximated with an empirical equation. Copyright © 2017 John Wiley & Sons, Ltd.