A combination of dynamic Monte Carlo (DyMC) and quantum calculation methods is proposed to estimate the diffusion coefficient and permeance in zeolite systems. Results are presented for the estimation of the diffusion coefficient and permeance of aromatic molecules in MFI-type silicalite and Mg ion-exchanged ZSM-5 intercrystalline/membranes. The parameter required for DyMC was obtained by performing a PM3 calculation. The predicted diffusion coefficients of xylene isomers, benzene and toluene are qualitatively consistent with previously reported results. The permeance of xylene isomers through the silicalite and MgZSM-5 membranes was calculated using a non-equilibrium DyMC. MgZSM-5 membranes exhibited a greater selectivity for p-xylene from among xylene isomers than that of the silicalite membranes. Results of the PM3 calculation provide evidence that the Mg ion functions as the attractive site for xylenes. (C) 2004 Elsevier B.V. All rights reserved.