Separation of complex gas mixtures, such as natural gas using polymer membranes, often involves components with a wide range of condensabilities and propensity to interact with the polymer, potentially changing the transport properties of all components. There are few studies of such phenomena in the open literature. Here, the influence of toluene, a surrogate aromatic contaminant in natural gas, on pure and mixed CO2 and CH4 gas transport properties at 35 degrees C was investigated for a thermally rearranged (TR) polymer prepared from a polyimide precursor based on 3,3'-dihydroxy-4,4'-diamino-bi-phenyl (HAB) and 2,2'-bis-(3,4-dicarboxy-phenyl) hexafluoropropane dianhydride (6FDA). As the polymer was exposed to CO2, CH4, and their equimolar mixture, at a toluene activity of about 0.3, CO2 and CH4 permeability coefficients decreased by more than 90% relative to their respective pre-exposure values due to antiplastization and competitive sorption. CO2/CH4 selectivity went through a maximum as toluene activity increased, reflecting the interplay between competitive sorption, antiplasticization, and plasticization. The recovery of gas permeation properties was explored by measuring CO2 and CH4 permeabilities after removing toluene from the feed. These effects were largely reversed when toluene was removed from the feed gas mixture. Toluene vapor sorption was determined as a function of toluene activity, and the sorption data were used to help rationalize the changes in CO2 and CH4 gas permeability coefficients in the presence of toluene. A qualitative assessment of antiplasticization and competitive sorption effects was provided using the partial-immobilization dual mode model. (C) 2016 Elsevier B.V. All rights reserved.