The oxygen permeability of dense La0.3Sr0.7CoO3−δ membranes has been measured in the range 750–1100°C under various oxygen partial pressure gradients. A sweep gas method was employed. Results indicate that in the range of thickness 0.057–0.215 cm used in the present study, the oxygen flux is predominantly controlled by bulk diffusion across the membrane. The measured activation energy is 60 kJ mol−1. By fitting the permeation data for various thicknesses to the transport equation obtained upon assuming linear kinetics for the surface exchange reactions and bulk ionic transport, we could derive the oxygen ionic conductivity and the characteristic membrane thickness. The latter quantity determines the transition from predominant control by diffusion to that by surface exchange. The ionic conductivity is about 0.5 S cm−1 at 1000°C. The characteristic thickness is extrapolated at a value of about 80 μm.