A new carbonate-ceramic dual-phase membrane for high temperature CO(2) separation was synthesized from porous La(0.6)Sr(0.4)Co(0.8)Fe(0.2)O(3-delta) (LSCF) supports infiltrated with a eutectic molten carbonate (Li(2)CO(3)/Na(2)CO(3)/K(2)CO(3)) mixture. Helium permeances of the LSCF support before and after infiltration of molten carbonate were found to be on the order of 10(-6) and less than 10(-10) mm m(-2) s(-1) Pa(-1) respectively. On average, supports were found to increase in weight by 33% after infiltration. High temperature CO(2) permeation experiments were carried out for several membranes of varying thickness under low oxygen partial pressure (P(O2) = 10(-4) atm). Maximum permeances of 2.01, 3.73, 4.63 and 4.77 x 10(-8) mol m(-2) s(-1) Pa(-1) were obtained at 900 degrees C for dual-phase membranes with thicknesses of 3.0, 1.5, 0.75 and 0.375 mm respectively. A CO(2)/Ar separation factor of at least 225 was achieved at 900 degrees C for the 0.375 mm thick membrane. The activation energy for CO(2) permeation for these membranes was calculated to be 86.4-89.9 kJ mol(-1) depending on membrane thickness. Experimental CO(2) permeation data can be well predicted by a model that considers oxygen ionic conduction in the solid phase of LSCF support in conjunction with structure data for the molten carbonate and LSCF solid phases that was characterized by helium permeation and electrical conductivity measurements. (C) 2010 Elsevier B.V. All rights reserved.