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In this study, we examine the parameters that govern the overall oxygen flux through a permeation membrane. The chemical diffusion (D) and the surface exchange (k) coefficients for oxygen in La0.6Sr0.4Fe0.8Co0.2O3-d were determined as a function of temperature using a specially designed electrochemical cell combined with impedance spectroscopy. Typically, D=1.7 10-5 cm2/s and k=3.6 10-4 cm/s in air at 900°C. These values were compared with literature 18O/16O isotope exchange data. Oxygen permeation measurements were also performed on the same material in an air/Ar gradient, in the temperature range of 800 to 1000°C. At 900°C, the oxygen flux across a 1.53 mm thick membrane was 8.0 10-8 mol/(cm2s). The measured fluxes were compared with fluxes calculated on the basis of the D and k values using expressions derived from Fick’s law. Comparison showed that the flux is controlled by both bulk diffusion and surface exchange, even for such thick membranes, and that the apparent k? varies significantly from one experiment to another.