We have carried out non-equilibrium molecular dynamics simulations of gas separation in a "selective surface flow" membrane. The gas mixture studied is hydrogen/methane, which is relevant to hydrogen purification in refineries. The simulations give insight into the separation mechanism, which is based on the transport of the more strongly adsorbing species (methane) in a dense layer near the pore wall, with the less strongly adsorbed species (hydrogen) diffusing through a less dense region close to the centre of the pore. Good agreement is obtained with experimental selectivity data. This work is also relevant to the study of the combined effects of adsorption and diffusion in microporous carbon adsorbents. (C) 2003 Elsevier Ltd. All rights reserved.