Dense La0.4Sr0.6Co0.2Fe0.8O3-δ oxygen-permeable membranes of different thickness were prepared by solid-state reaction. The oxygen flux facilitated by an oxygen partial pressure difference across the oxide membrane was measured at elevated temperatures. It is revealed that oxygen permeation flux is entirely controlled by the transport of oxide ions in the bulk of the membrane in the thickness range of 1.25−2.46 mm. When the thickness is decreased to 0.62 mm, it becomes jointly controlled by the oxygen exchange across the gas and solid interfaces. An electrochemical approach is adopted to analyze the thickness dependence of the oxygen flux. It is shown that the interfacial process is more sensitive to the change of temperature, as evidenced by the larger activation energy for the surface oxygen exchange (188.2 ± 2.2 kJ mol-1), than that for the bulk transport (93.7 ± 1.7 kJ mol-1).