A novel method for the determination of the H/D isotope effect in hydrogen solubility in proton-conducting oxides is proposed. The method is based on comparison of the chemical expansion under heavy and regular water vapor pressures. The theoretical approach has been developed for acceptor-doped perovskites with the potential extension to other nonstoichiometric oxides. Dilatometric measurements of the expansion of BaZr0.9Y0.1O3-delta ceramic samples equilibrated with the atmospheres containing H2O or D2O vapors have been performed. The ratio of concentrations of protons and deuterons incorporated under equal H2O and D2O pressures and other identical external conditions have been found. The observed isotope effect in hydrogen solubility is substantial, its magnitude and.behavior agree with our theoretical predictions for proton-conducting oxides. (C) 2017 Elsevier B.V. All rights reserved.