In this paper, we systematically studied the effects of membrane thickness, CO2 concentration, and operating time on the CO2 flux density of an Al2O3-surface-moclified and carbon-black-pore-former derived silver-carbonate membrane, It was found that the CO2 transport through a silver-carbonate membrane exhibited a critical thickness of 0.84 mm, below which no flux enhancement could be achieved. The CO2 flux was also observed to be proportional to the difference of (PCO2PO21/2) at the two reacting surfaces, and not the commonly assumed logarithmic partial pressure difference. The CO2 flux tested against time at 650 degrees C increased by 200% for the first 160 h, followed by a gradual decrease. At the 326-h marker, the flux was still the same as the original value when the test was started. Overall, the use of carbon black as a pore-former and Al2O3 as a surface modifier for silver-carbonate membranes has been proven effective and necessary to achieve a higher and more stable CO2 flux density. 2014 Elsevier B.V. All rights reserved.