Carbon molecular sieves (CMS) are valuable materials for the separation and purification of gas mixtures. In this work, plasma deposition was used aiming to the formation of pore constrictions, by narrowing the surface pore system of commercial activated carbon fibers (ACF). For this reason propylene/nitrogen or ethylene/nitrogen discharges of 80 and 120 W were used. The molecular sieving properties of the plasma treated ACF were evaluated by measuring the adsorption of CO2 and CH4. The CO2/CH4 selectivity was significantly improved and depended on plasma treatment conditions (discharge gas and power). The Optimum CO2/CH4 selectivity (26) was observed for C2H4/N-2 plasma treated ACF at 80 W. Sample scanning electron microscopy (SEM) analysis after plasma treatment revealed an external film formation and X-ray photoelectron spectroscopy (XPS) analysis showed the incorporation of nitrogen functional groups in the film, which probably interact with CO2, thereby altering CO2/CH4 selectivity. (C) 2003 Elsevier Ltd. All rights reserved.