An analysis is presented of simultaneous carbon dioxide and hydrogen sulfide transport in immobilized liquid membranes comprised of alkali carbonate solutions. Emphasis is placed upon membranes so thin that reaction effects upon CO2 transport are negligible, as a result of which permselectivity for H2S can be quite large. Comparison is made with experimental data obtained to simulate acid gas separation downstream of coal gasification. Results indicate that, because of membrane thickness, selectivity was compromised by facilitation of CO2 transport.