The electrical properties of (CZY) were studied as a function of both oxygen partial pressure and water vapor partial pressure in the temperature range of and the partial conductivities of protons, holes, and oxygen vacancies were calculated from the defect model. In a wet atmosphere, CZY was a mixed conductor of protons, holes, and oxygen ions. No conduction transition from protons to holes and/or oxygen ions occurred even as temperature was increased up to . The calculated activation energy of proton transport was . The standard solution enthalpy for water dissolution was best fitted with a slope of . The twofold conductivity relaxation behavior that was observed at during hydration/dehydration was interpreted in terms of decoupled ambipolar diffusion of oxygen vacancies and protons by holes. The best-estimated chemical diffusivities of oxygen and hydrogen were several orders of magnitude lower than that reported for other proton-conducting perovskite oxides due to the relatively low ion conductivity of CZY.