We report the development of a dual-phase membrane for CO2 separation based on a molten hydroxide liquid phase and a nanoporous yttria-stabilized zirconia solid support phase, termed hydroxide/ceramic dual-phase (HCDP) membranes, that can operate in the flue gas temperature range of 250-650 degrees C. HCDP membranes demonstrate selectivity for CO2 over N-2 greater than 1000 and CO2 permeability of 1.78 +/- 0.16 x 10(-10) mol m m(-2) s(-1) Pa-1 (5.32 x 10(5) barrer) at 550 degrees C with 20 vol% CO2, which is more than an order of magnitude greater than the best values for dual-phase and polymer membranes reported in the literature. We also demonstrate using three different experimental methods that CO2 absorption by molten hydroxides is reversible in the presence of water vapor at temperatures as low as 300 degrees C.