The permeation properties of a mixed matrix membrane film were investigated with 10% CO2/90% CH4 gas feeds containing a vapor impurity of toluene (70 ppm). The mixed matrix film was comprised of fine particles of high-selective carbon molecular sieves (CMS) dispersed within a glassy polyimide matrix (Matrimid(R) 5218). In preceding publications, it was demonstrated that the carbon molecular sieve (CMS) particles produced a mixed matrix membrane having significantly enhanced effective permselectivities (CO2/CH4 and O-2/N-2) over the intrinsic properties of the neat polymer matrix phase alone. In this paper, a Matrimid(R)-CMS mixed matrix flat film (19 vol.% CMS) was exposed to a 10% CO2/90% (CH4/70 ppm toluene gas feed at 500 psia (34.5 bara) and 35 degreesC for a period of up to 60 h. The identical experiment was performed on a pure Matrimid(R) polymer film for comparison. The results of this short-term study indicate that both the Matrimid(R) mixed matrix film and the pure Matrimid(R) film exhibit fairly stable properties in the presence of the low-concentration toluene impurity. The Matrimid(R)-CMS mixed matrix film (19 vol.% CMS) sustains its CO2/CH4 selectivity enhancement (similar to 13%) over the intrinsic CO2/CH4 selectivity of the pure Matrimid(R) polymer film. As speculated in a previous publication on CMS fibers, larger-sized impurities, such as toluene, may only be successful in blocking or occupying the larger, non-selective pores of the CMS particles and may not access the smaller, selective pores that are accessible to C02. In the presence of the binary mixture of 10% CO2/90% CH4, permeabilities approach a stable steady state within a few hours for both the pure Matrimid(R) polymer film and the mixed matrix sample. For the pure Matrimid(R) polymer film, the C02 permeability in the presence of the ternary mixture is slightly lower, but barely distinguishable beyond experimental uncertainty. On the other hand, the mixed matrix sample shows a more protracted approach to steady state in the presence of the toluene impurity, which ultimately stabilizes after roughly 60 h. We hypothesize that surface adsorption of the toluene molecules onto the CMS particles may induce a slow relaxation leading to better packing at the interface between the carbon and matrix reflected by the protracted changes seen. (C) 2003 Elsevier B.V. All rights reserved.