Measurements of oxygen flux, in the temperature range, 770-870 degrees C on planar membranes of SrCo0.8Fe0.2O3-delta, consisting of a dense discs similar to 0.1 cm thick modified by coating with porous layers on both sides, yielded an enhancement in the oxygen flux, eta approximate to 2, compared to the uncoated membrane. When only one side of the disc was coated with the porous layer on the high pressure side of the membrane then eta approximate to 1.2, while when the modified side was on the low pressure side then eta approximate to 1.4. The observed values of eta were explained by a model of diffusion-reaction transport in mixed ionic electronic conductor membranes with porous electrodes, extended to the case of finite thickness of the dense layer. From a fit of the model to the experimental data, values of the surface exchange coefficient, k(iO), and ambipolar oxygen ion-electron hole pair diffusion coefficient, D-a, were deduced. At 870 degrees C, k(i0) = 2.5 x 10(-4) cm/s and D-a = 5 x 10(-6) cm(2)/s.