This paper describes a new concept of dense, dual-phase membrane consisting of two phases conducting, respectively, CO32- and electrons for selective permeation of CO2 and O-2 at high temperatures. Membranes with a molten carbonate phase in a porous stainless-steel support were synthesized by a direct infiltration method. Membrane preparation conditions were optimized to obtain stable, gas-tight dual-phase membranes at high temperatures (> 450 degrees C). The dual-phase membranes exhibit low single-gas permeance for pure CO2 and N-2 (< 5 x 10(-9) mol s(-1) m(-2) Pa-1) in 450-650 degrees C but substantially higher CO2 permeance for CO2 mixed with O-2 at high temperatures. CO2, after interacting with O-2, transports through the molten carbonate phase in a form of CO3, with electrons transporting through the porous metal support. With further improvement, the dual-phase membranes may offer application in producing O-2-enriched CO2 streams for the oxyfuel combustion process. The concept can be also extended to prepare CO2 perm-selective dense membranes for high-temperature CO2 separation.