A non-equilibrium molecular dynamics (NEMD) technique has been utilized to simulate permeation of pure and mixed gases across ZSM-5 membranes. Propane and propylene are chosen as permeating gases. The ideal separation factor and the permselectivity of propylene to propane calculated from simulations are in good agreement with the experimental data though the permeabilities from simulations are about one order higher than those of experiments. For pure gas permeation, permeability of propane is smaller than that of propylene, while for mixed-gas permeation, it becomes larger than that of propylene due to the competitive adsorption of propane. The present simulations have suggested that the permeation mechanism of propane and propylene across ZSM-5 membranes is selective adsorption followed by diffusion in zeolite pores. (C) 2003 Elsevier Science B.V. All rights reserved.