By means of a high-temperature gravimetry, the defect chemical relationships between oxygen nonstoichiometry and water content in BaCe0.9M0.1O3-delta, (M=Y and Yb) were investigated as functions of.. partial pressure of oxygen, P(O-2), partial pressure of water vapor, P(H2O) and temperature Concentrations of,. protonic defect and that of oxygen vacancy strongly depend on P(H2O) and temperature, while the dependences on P(02) were weak. The equilibrium constants of the water vapor incorporation reaction H2O + V-0(*) + O-0(x) = 2OH(o)* determined. Concentrations of hole, [h*], in the dry-atmospheres were determined by the weight gain by the incorporation of oxygen from the gas atmospheres. The [h*] values increased with decreasing temperature. The [h*] values were estimated to be about 2 to 3 orders of magnitude less than [OHo*] values measured in the wet-atmospheres. (C) 2008 Elsevier B.V. All rights reserved.