A membrane device that can selectively separate CO2 at temperatures exceeding 600 degrees C has been demonstrated. The membrane can be made from a composite material made of a molten carbonate electrolyte that fills the pore space in a solid oxide electrolyte (e.g. yttria doped zirconia (YSZ), or gadolinia doped ceria (CGO)). The experimental evidence points to a transport mechanism based on opposing ionic currents of carbonate and oxide ions. The flux of CO2 across these membranes has been shown to increase with temperature, reaching permeabilities of 10(-11) mol m(-1) s(-1) pa(-1) (or permeance of 3 x 10(-8) mol m(-2) s(-1) Pa-1) at 850 C. The use of a non-ion conducting solid oxide, Al2O3, does not result in strong CO2 permeability or selectivity, supporting a facilitated dual-ion transport mechanism. (C) 2010 Elsevier B.V. All rights reserved.