High temperature membranes for CO2 separation could lead to more efficient energy conversion systems and more effective means of CO2 capture in power plants. One concept for a membrane that can separate CO2 at high temperatures from a gas mixture is described here. A theoretical model is presented to describe the steady state flux of CO2 through a mixed conducting, dual-phase membrane consisting of solid oxide and molten carbonate phases. An analytical flux solution is derived for the case in which only ionic conduction is present. The limiting factor dominating the flux of CO2 is the oxide ion conductivity. To take advantage of solid oxide materials with higher ionic conductivities, the additional presence of n-type electronic conductivity under low oxygen atmospheres is also investigated. Using numerical and perturbation techniques we show that the presence of electronic conduction will never be great enough to overwhelm the CO2 separation mechanism. (C) 2007 Elsevier B.V. All rights reserved.