The gas permeation properties from thick (bulk) films have long been used as a guide to the performance of asymmetric membranes with a very thin selective skin (similar to 100 nm). However, recent research has shown that thin films of glassy polymer with a similar thickness undergo more rapid physical aging than such thick films, leading to different permeation characteristics. Our prior work has dealt with pure gases, and most recently, pure CO2 has been used to explore the difference in plasticization behavior for thick and thin films. This paper is the first effort to look at mixed gases involving CO2 and how thin films differ from thick films. Thick and thin films made of Matrimid, an important polymide for commercial gas separation membranes, have been investigated by monitoring their gas permeability and selectivity simultaneously for CO2/CH4 and CO2/N-2 gas mixtures. The effects of film thickness, aging time, pressure, gas pair and gas composition on mixed gas behavior have been discussed in detail. The pressure dependence of pure gas permeability has also been measured as a control. Matrimid films become more vulnerable to CO2 plasticization as the thickness is reduced to the range of thin selective skins of asymmetric membranes and with increasing aging time. Competitive sorption/permeation effects appear to be more pronounced and the combined effect of CO2 plasticization and competitive sorption/permeation on thin Matrimid films seems to be more severe for CO2/CH4 feed mixtures than for CO2/N-2 mixtures. (C) 2013 Elsevier B.V. All rights reserved.