Proton concentration in BaZr0.9Y0.1O3 − δ was measured by highly-accurate thermogravimetry as functions of temperature and p(H2O), and thermodynamic properties of the proton dissolution were discussed. Apparent Gibbs free energy change of the proton dissolution reaction changes with proton concentration even at a constant temperature. This means the proton dissolution in BaZr0.9Y0.1O3 − δ is not ideal-solution-like. The deviation from the ideal solution became more significant with increasing temperature i.e. decreasing the proton concentration. This indicated the interaction between oxygen vacancies or electron hole, dopant-defect interaction, or lattice deformation may cause the proton dissolution in BaZr0.9Y0.1O3 − δ especially at higher temperature.