To clarify the electrochemical properties of Al-doped CaZrO3, the electrical conductivity of the polycrystalline specimen was measured by employing the two-terminal AC method. By examining the H/D isotope effect of the electrical conductivity, it was found that protons are the dominant charge carrier at temperatures from 873 to 1373 K. both in hydrogen-rich and oxygen-rich atmospheres. At higher temperatures of 1373-1673 K, the atmospheric dependence of the electrical conductivity shows that the dominant charge carrier is the positive hole in oxygen-rich atmospheres, whereas it is the oxide ion vacancy in hydrogen-rich atmospheres. (C) 2010 Elsevier Ltd. All rights reserved.