Asymmetric dual-phase composite membranes for oxygen separation were conveniently fabricated by an acid leaching technique. A thin dense layer of Ce0.85Sm0.15O1.925/Sm0.6Sr0.4FeO3-delta was left by controlling the degree of acid leaching, and a porous substrate of Ce0.85Sm0.15O1.925 with a fluorite structure was formed after dissolution of Sm0.6Sr0.4FeO3-delta with a perovskite structure in HCl. Thus, a thin dense layer and a porous substrate can be fabricated in a single step in which traditional shrinkage mismatch and chemical reaction between thin dense layers and porous substrates can be avoided. The thickness of the dense layer can be controlled by varying the acid leaching time. Hence, dual-phase composite membranes with high oxygen flux can be obtained. (c) 2008 Elsevier B.V. All rights reserved.