CO2-stable oxygen-permeable Fe2O3 (FO) - Ce0.9Gd0.1O2-delta (CGO) dual phase composite membranes of the composition chi wt % FO - (100 - chi) wt % CGO with chi = 25, 40, 50 were successfully prepared via a one-pot single-step method. X-ray diffraction (XRD) demonstrated that all FO -CGO composite membranes after sintering at 1300 degrees C for 5 h represent a microscale mixture of only the two pure phases FO and CGO. Scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDXS), and oxygen permeation revealed that the microstructure of the composition of the chi FO - (100 chi) CGO dual phase membranes has a great influence on the oxygen permeability including its time-dependence. It was found that the composition of 40FO - 60CGO displays the highest oxygen permeability. An oxygen permeation flux of 0.18 mL/min.cm(2) was obtained through the uncoated 40FO - 60CGO membrane with a thickness of 0.5 mm under an air/He oxygen gradient at 1000 degrees C. In situ XRD demonstrates that the 40FO -60CGO material possesses a good phase stability not only in an atmosphere of 50 vol % CO2/50 vol % Ar but also in other atmospheres with a low oxygen partial pressure like reduced pressure (vacuum) and 5 vol % H-2/95 vol 96 He. After coating the 40FO - 60CGO dual phase membrane with a porous La0.6Sr0.4CoO3-delta (LSC) layer of a few pm thicknesses on the air side, the oxygen permeation flux reaches the steady state immediately. This steady oxygen permeation flux of the LSC coated membrane was found to be 0.20 mL/min.cm(2) unchanged for more than 150 h even when pure CO2 was used as the sweep gas) which indicates that the coated 40FO - 60CGO dual phase membrane is CO2 stable.