The trajectories of carbon dioxide, oxygen and nitrogen in membranes prepared from poly(bisphenol A-co-4-nitrophthalic anhydride-co-1,3-phenylene diamine) were simulated by assuming that the molecules of gas migrate through the glassy polymer in a sequence of hops between local minima of the potential energy. The values of the diffusion coefficients in units of 10⁻⁹ cm² s⁻¹ were 1.8, 7.3 and 7.8, in fair agreement with the experimental results, 1.3, 5.4 and 1.6, respectively, in the same units. The solubility coefficient was simulated by using Monte Carlo techniques in which the probabilities of inserting/removing a molecule of diffusant in a matrix containing n molecules were evaluated as the product of three factors depending, respectively, on: (a) the pressure p and temperature T, (b) the interaction energy between the diffusant particle and the polymer host matrix, and (c) the volume of the diffusant. The simulated pressure dependence of the concentration of carbon dioxide in the membrane displays the same pattern as that experimentally found. The simulated values of the solubility coefficients of CO₂, O₂ and N₂, at 30 °C and 76 cmm Hg of pressure, are in rather good agreement with the experimental results obtained from both permeation and sorption experiments.