To explore oxygen permeable materials, oxygen permeation properties of partially A-site substituted BaFeO3-delta perovskites were investigated. Ba sites in BaFeO3-delta were substituted with cations such as Na, Rb, Ca, Y, and La by 5%. The partial substitution with Ca, Y, and La, whose ionic radii are smaller than that of Ba, succeeded in stabilizing a cubic perovskite structure that is a highly oxygen permeable phase, as revealed by X-ray diffraction analysis. This can be explained in terms of a decrease in the tolerance factor (t). Among the Ba0.95M0.05FeO3-delta (M = Na, Rb, Ca, Y, and La) membranes tested, Ba0.95La0.05FeO3-delta showed the highest oxygen permeability at 600-930 degrees C, owing to the stabilization of the cubic phase without the formation of impurity phases. From chemical analysis, the oxygen permeability of Ba1-xLaxFeO3-delta membranes was correlated with the amount of oxygen defects (delta) in the lattice. The oxygen permeation flux of Ba0.95La0.05FeO3-delta membrane was significantly increased by reducing its thickness. Furthermore, a Ba0.975La0.025FeO3-delta membrane exhibited good phase stability under He flow at elevated temperatures. The obtained results indicate the promising properties of Ba1-xLaxFeO3-delta membranes as a cobalt-free material that has a high oxygen permeability, good phase stability, and low cost. (C) 2009 The Electrochemical Society. [DOI: 10.1149/1.3231690] All rights reserved.