Oxygen permeable ceramics based on mixed conductors are attracting much attention for use in partial oxidation of hydrocarbons as a novel technique for syngas and pure hydrogen production. This paper describes the preparation and oxygen permeation properties including the methane reforming property of a novel member of oxygen permeable ceramics. The materials used are solid solutions of . The single phase of perovskite-type is obtained in the range of to 0.9. The highest oxygen flux densities of 2.2 and (membrane thickness, ) are attained for at under He/air and /air gradients, respectively. The electrical conductivity of is dominated by -type conduction having a slope of under the high region. The oxide-ion conductivity of the same sample is estimated to be at . Even though the oxygen flux density slightly decreases with increasing time, high CO selectivity of 90% is kept for . The oxygen flux density of the solid solution is also discussed in the context of surface exchange kinetics.