Permeation of mixtures of butane isomers through 6FDA-DAM (2,2-bis(3,4-carboxyphenyl) hexafluoropropane dianhydride-diaminomesitylene) dense films is shown to deviate from the dual-mode transport model, even if one accounts for nonideal bulk flow effects. Two hypotheses are proposed to explain the deviations observed. Both hypotheses relate to an apparently much slower exchange rate of the bulky i-butane molecule versus n-butane isomer between Henry's Law and Langmuir sorption sites in the rigid matrix. The first hypothesis suggests a change in the equilibrium isotherm for normal butane, due to the presence of i-butane. A model is considered to account for the change in local equilibrium and to improve the description of experimental data compared to the dual-mode transport model accounting for bulk flow. The second hypothesis considers an effect on the accessible unrelaxed volume associated with the Langmuir capacity of one penetrant due to the presence of the second penetrant. We introduce an empirical parameter representing this effect for each penetrant in the n/i-butane system. Models based on either hypothesis show some improvement to the dual-mode transport model accounting for bulk flow, in making mixed gas predictions; however, the second model gives a much better fit to experimental data. Further experimentation is required to provide a physical basis for the empirical factor in the second model, thereby allowing generalization of the observations reported here. (C) 2011 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 49: 1605-1620, 2011