Oxygen permeable composite ceramic membrane of 75 wt.% Ce(0.8)G(0.2)O(1.9) wt.% Gd0.2Sr0.8FeO3-delta (GDC-GSF) was investigated to understand the relationship between homogeneity and oxygen permeability. Four different methods were employed for its preparation, i.e. (1) mixing powders method, (2) packing method, (3) loading method and (4) one pot method. Structures and reactions between the two oxides were investigated by X-ray diffraction (XRD), backscattered electron microscope (BSEM) and local areas energy-dispersive X-ray spectra (EDXS) analysis. XRD results showed that it was compatible between the fluorite phase and the perovskite phase in the composite membranes synthesized by all the methods. BSEM results revealed that different preparation methods would lead to different homogeneity of two phases in composite membranes and the membrane derived from one pot method showed the most uniform mixture of the fluorite phase and perovskite phase. The local areas EDXS analysis showed minor inter-diffusion of elements between GDC and GSF. Oxygen permeation measurements indicated that the inter-diffusion has little effects on the oxygen permeability and showed that a more uniform of the microstructure would result in higher oxygen permeability. The concept of three-phase boundaries (TPB) was proposed to understand the oxygen exchange reactions on membrane surfaces. TPB concept and the block of fluorite grains on electrons transport between perovskite grains was used to explain the relationship between the homogeneity and the oxygen permeability of composite membranes. (C) 2007 Elsevier B.V. All rights reserved.