The oxygen permeation and stability of La(0.2)A(0.8)Co(0.2)Fe(0.8)O(3-delta) (A = Sr, Ba, Ca) perovskite-type membranes were studied at high temperatures and low oxygen partial pressures. The oxygen vacancy diffusivity and concentration gradient calculated from the unsteady-state and steady-state oxygen data for the three membranes decrease in the order of Sr > Ba > Ca. The activation energies for oxygen permeation increase in the order of Sr < Ba < Ca. The oxygen permeation data of the three membranes can be explained by the average bond energy, free volume, and critical radius of the materials. After exposure to air at 1173 K for 10 h, part of La0.2Sr0.8Co0.2Fe0.8O3-delta transforms to La2O3, SrO, CoO, and Fe and the crystallite size of the perovskite phase decreases. These changes were not observed for La0.2Ba0.8Co0.2Fe0.8O3-delta under the same conditions, indicating that La0.2Ba0.8Co0.2Fe0.8O3-delta membrane is much more stable than La0.2Sr0.8Co0.2Fe0.8O3-delta at high temperatures and low oxygen partial pressures.