A mathematical formulation for a laminar co-current gas absorption process was developed in a parallel-plate gas-liquid membrane contactor that independently regulates gas and liquid flows. Physical carbon dioxide absorption in water was studied. The analytical solutions for such a conjugated Graetz problem were solved for fully developed laminar velocity profiles using the orthogonal expansion technique in power series. This work investigates the influence of the absorbent flow rate, gas feed flow rate and the CO2 concentration in the gas feed on absorption efficiency. The theoretical predictions show that the initial CO2 concentration effect in the gas feed on the absorption efficiency is more significant than that from the gas feed flow rate variation as the absorbent flow rate is specified. A higher absorbent flow rate has a positive effect producing better absorption efficiency in the case when the CO2 concentration in gas feed is specified. (c) 2005 Elsevier B.V. All rights reserved.