In order to understand the large solubility of protons in perovskite proton conductors, a new approach employing the hydroxyl capacity, termed C-OH' = [OHO']. [V-O"](-1) . p(H2O)(-1/2), is proposed and examined for 5 mol% YbO1.5-doped BaCeO3. Prior to the solubility experiments, the phase relation in an isothermal section of the phase diagram for BaO-CeO2-YbO1.5 was determined and activity measurements for BaO were carried out. The values of C-OH' were evaluated by measuring the solubility of water using the thermogravimetric method under a controlled BaO activity by employing the reaction BaO(in BuCeO3) + CO2(g) = BaCO3. C-OH' did not exhibit a strong dependency on the BaO activity, indicating that the basicity of BaCeO3, defncd as the activity of O2- ions, is almost constant across the homogeneity region. It has been estimated that the A-site vacancy in ABO(3) mainly contributes to the steep change in BaO activity.