In this study, A-site defective CayTi0.93Sc0.07O3-α oxides were prepared to examine their ionic conduction properties.The elec. conductivities of two typical compositions were measured as functions of oxygen partial pressure PO2, temperature, and humidity.Addnl., phase transition, chem. expansion, and CO2 tolerance were examined in Ca0.985Ti0.93Sc0.07O3 using an atm.-controlled high-temperature X-ray diffraction.In Ca0.947Ti0.93Sc0.07O3, the ionic conduction domain over a wide range of PO2 was observed at 500-800 °C, even though the humidity dependence of conductivities was confirmed only at 500 °C.Conversely, in Ca0.985Ti0.93Sc0.07O3, the conductivities were enhanced in humidified atmospheres at 500-800 °C, while the ionic conductivities in dry atmospheres were higher than those of 8YSZ.As protonic and oxide ionic conductivities are comparable, the proton-oxide ion mixed conduction can be considered to occur in Ca0.985Ti0.93Sc0.07O3-α.Therefore, a small percentage of Ca defect in CayTi0.93Sc0.07O3-α affects not only conductivity but also conductive ionic species.Furthermore, Ca0.985Ti0.93Sc0.07O3 did not show any phase transition and chem. expansion with hydration up to 900 °C.The crystal phase of Ca0.985Ti0.93Sc0.07O3 during the CO2 tolerance test was observed to be stable.Therefore, the material properties of CayTi0.93Sc0.07O3-α suggest its high potential as electrolytes in high temperature electrochem. devices.