The electrical properties of Y-doped barium cerate (BaCe0.85Y0.15O3-delta; BCY) were studied as a function of both oxygen partial pressure (-3.34 <= log P-O2/atm <= -2.72) and water vapor activity (-3.34 <= log P-H2O/atm <= -2.72) in the temperature range of 700-800 degrees C. The total conductivity slightly increased in reducing atmospheres with increasing water vapor activity but tended to be independent as the atmosphere became oxidized due to the relative contribution to the total conductivity by redox reaction at the given thermodynamic conditions. Due to the sluggish H-unmixing in the oxygen chemical potential gradient and the negligible hydrogen chemical potential gradient developed during oxidation and reduction at a fixed water vapor activity, the chemical diffusivity of oxygen at a fixed water vapor activity, (D) over tilde (vO), could only be evaluated from Fick's second law. The chemical diffusivity of oxygen for BCY was an order of magnitude higher than that observed in the Yb-doped SrCeO3 proton conductor due to the higher proton conductivity of BCY. (C) 2011 The Electrochemical Society. [DOI: 10.1149/1.3597634] All rights reserved.