Permeability coefficients for pure CO2, pure CH4, and CO2/CH4 mixtures containing 50% CO2 are reported for a polyimide synthesized from 3,3'dihydroxy-4,4'-diamino-biphenyl (HAB) and 2,2'-bis-(3,4-dicarboxyphenyl) hexafluoropropane dianhydride (GFDA) and for three thermally-rearranged (TR) derivatives thereof. Permeability measurements were made at 35 degrees C for fugacities ranging horn 4 to 25 atm. The permeability of CO2 and CH4 increased as the degree of TR conversion increased. For example, CO2 permeability at 10 atm increased by a factor of 30 between the unconverted polyimide and its TR analog converted at 450 degrees C. In pure-gas experiments, CO2 was observed to plasticize the unconverted polyimide, but it did not appear to plasticize the TR polymers in mixed-gas experiments, dual-mode competitive sorption caused a depression in CH4 permeability, with very little change in CO2 permeability. In addition, plasticization by CO2 was evident in the CH4 mixed-gas permeability trends, but its impact was small in contrast with dual-mode competitive effects. Consequently, CO2/CH4 mixed-gas permeability selectivity was higher than the ideal selectivity, calculated as the ratio of pure gas permeability coefficients. The dual-mode sorption and permeation model was fit to the experimental data Dual-mode model parameters and model predictions are reported, along with their confidence intervals. By comparing the dual-mode model predictions with the experimental mixed-gas data, the degree of CO2-induced plasticization was observed to decrease as the degree of TR conversion increased and was completely absent (within experimental uncertainty) for the TR polymer converted at 450 degrees C. (C) 2014 Elsevier B.V. All rights reserved,