We consider the adsorption and diffusion of two gas species, A and B. While the presented theoretical model is qualitative and all the results are presented in arbitrary dimensionless units, several parameters associated with these two species are guided by the underlying problem of CO 2 and N 2 separation. Species A represents CO 2 and all interactions in the system are considered in units of fluid-fluid interactions for species A. The ratio of fluidfluid interactions for species A and B is equal to the ratio of CO 2 and N 2 critical points. The model then effectively has only two free parameters: the strength of solid-fluid interactions and the volume fraction of the solid lattice sites, ϕ s . To represent membranes with different polymer-fluid interactions, three combinations of solid-fluid interaction strengths are considered. In particular, in C 1 case both fluids interact strongly with the polymer; in C 2 case both fluids interact weakly with the polymer; and in C 3 case fluid A interacts strongly with the polymer, while B is weakly interacting. The actual values of the parameter ϵ sf describing solid-fluid interactions for species f are a result of some preliminary experimentation with the model and are somewhat arbitrary. However, what is more important is the range of adsorption and transport scenarios emerging as a result of different interaction models.