A composite of oxygen ion conducting oxide Ce0.8Sm0.2O2−δ (60 vol.%) and electron conducting oxide La0.8Sr0.2CrO3−δ was prepared by sintering a powder compact at a temperature of 1550 °C. No significant reaction between the two constituent oxides was observed under preparation and oxygen permeation conditions. Appreciable oxygen permeation fluxes through the composite membrane were measured at elevated temperatures with one side of it exposed to the ambient air and the other side to a flowing helium gas stream. The oxygen flux initially increased with time, and took a long time to reach a steady value. A steady oxygen permeation flux as high as 1.4 × 10−7 mol cm−2 s−1 was obtained with a 0.3 mm thick membrane at 950 °C under a relatively small oxygen partial pressure difference of 0.21 bar/0.0092 bar. It was revealed that the overall oxygen permeation process was mainly limited by the transport in the bulk of the membrane in the range of the membrane thickness greater than not, vert, similar1.0 mm, and the limitation by the surface oxygen exchange came into play at reduced thickness of not, vert, similar0.6 mm.