Oxygen permeation fluxes have been investigated as a function of temperature for a mixed ionic-electronic conducting La0.6Sr0.4Co0.2Fe0.8O3 (- delta) (LSCF) and La0.7Sr0.3 Ga-0.6 Fe-0.4 O3 - delta (LSGF) membranes. Ga-doped composition, which is known for its chemical and structural stability, shows limited oxygen permeation flux as compared with a Co-containing system. However, modification of both surfaces with catalytically surface-reactive La0.6Sr0.4CoO3 - delta (LSC) makes Ga-doped perovskite an excellent oxygen-permeable membrane. It has been demonstrated that the effective area of reactive free surface is an important factor in determining the effectiveness of surface modification by La0.6Sr0.4CoO3 - delta On the contrary, the oxygen permeation flux of La0.6Sr0.4Co0.2Fe0.8O3 - delta is not affected by surface modification. The rate-determining process is discussed in conjunction with the apparent activation energy for oxygen permeation. As cationic substitution occurs in the vicinity of the interface between the coating layer and substrate membrane at elevated temperatures, the intermediate composition, that is, (La1-xSrx)(CoyGayFe1-y-y)O3-delta, has been formed in La0.7Sr0.3Ga0.6Fe0.4O3 (- delta) membrane surface-modified by La0.6Sr0.4CoO3 (-) delta.