The defect structure of BaZr0.85Y0.15O3-delta was systematically analyzed as functions of oxygen and water vapor activity at the temperature where the grain boundary resistance becomes negligible for defect-chemical analyses. AC impedance spectroscopy was used to differentiate the grain and grain boundary contribution on total electrical conductivity. A thermo-gravimetric analysis was employed to estimate the equilibrium constant of hydration reaction as well as the concentrations of ionic charge carriers, protons and oxygen vacancies. Total electrical conductivity was measured by a DC 4-probe method at 800 degrees C over a range of a(O2) from 10(-25) to 1 and a(H2O) from 10(-4.5) to 10(-1.6) and was analyzed via a non-linear simultaneous fitting method based on previously obtained proton and oxygen vacancy concentrations, to represent the corresponding partial ionic and electronic conductivities in the form of f(a(O2)center dot a(H2O)) within a three dimensional space. Transference numbers of each mobile charge carrier are also presented into the three-dimensional form. (C) 2011 Elsevier B.V. All rights reserved.