Extended to non equilibrium systems the successful analytical approach of density functional theory provides a tool to study what factors determine high selectivity in gas separation by nanoporous membranes. Based on a consideration of the most probable molecular trajectories, the method was originally developed for basic applications in non equilibrium systems and entails a rigorous derivation of the probability distribution required to calculate non equilibrium averages and system properties. The theory is applied to the calculation of the single gas permeation through a model nanopore. The interactions between gas particles and with the pore walls modify simple diffusive transport and are a possible cause of large variations of separation factor with applied pressure gradient. For a gas, which interacts weakly with the molecules of the pore, the effective diffusivity and the adsorption are enhanced with increase in loading and the permeance increases with pressure. For strong gas/pore wall interaction, the interactions within the gas compete with adsorption and the permeance decreases with increase in loading. The gas/wall interactions also switch the temperature dependence of the permeance at low density. The analytical results can be judged against numerical simulation with identical model parameters and when used together, help to achieve a better understanding of experimental trends. (C) 2005 Elsevier B.V. All rights reserved.