A composite membrane, which comprises of one fluorite oxide phase (Ce0.8Gd0.2O1.9) for oxygen ionic transport and one perovskite oxide phase (Gd0.2Sr0.8FeO3-delta) for both oxygen ionic and electronic transport, was investigated. XRD results revealed that the two oxides are compatible and showed good oxygen permeation stability at 950 C (more than 1100 h). Oxygen flux of the composite membrane was two times higher than that of the Gd0.2Sr0.8FeO3-delta mixed conducting membrane at the same conditions. At steady state, oxygen flux of a 0.5 mm membrane was 0.80 ml/(cm(2) min) under oxygen partial pressure gradient of air/He and similar to 5.0 ml/(cm(2) min) for syngas production at 950 degrees C, respectively. After similar to 440 h operation under syngas conditions, SEM analysis revealed that the syngas side of the membrane still kept dense and the EDX showed the transfer of elements occurred in the depth of a few microns. (c) 2008 American Institute of Chemical Engineers.