CO 2 permeation through an alkali carbonate-based membrane is interpreted as a combined absorption-desorption process driven by alkali ion diffusion in which the membrane material is actively involved by dissociation and re-formation. The relationships governing the gas permeation fluxes are derived for the most general case, on which basis the conditions for maximizing the CO 2 -to-O 2 separation factor are discussed in terms of the electronic conduction properties of the membrane material.