The electrical and relaxation properties of Y-doped barium cerate (BaCe0.85Y0.15O3-delta; BCY) were studied as a function of both P-O2 and P-H2O in the temperature range of 600-900 degrees C. The partial conductivities of protons, holes, and oxygen vacancies were successfully calculated, and the activation energy determined for proton transport was 0.29 eV. Twofold non-monotonic conductivity relaxation behaviors were clearly confirmed in the temperature range of 700 similar to 800 degrees C during hydration/dehydration. The hydrogen chemical diffusivity at a fixed oxygen partial pressure (P-O2), (D) over tilde (iH), was greater than that of (D) over tilde (vH), i.e., the oxygen chemical diffusivity at a fixed P-O2, at all experimental conditions, suggesting that the hydrogen chemical diffusion is always faster than oxygen upon hydration/dehydration in the temperature range studied. Copyright (C) 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.