Carbon molecular sieves (CMSs) have been incorporated into two different polymer matrices to form mixed matrix membrane films for gas separations. The CMSs were formed by pyrolysis of a polyimide (Matrimid((R))) precursor to a final temperature of 800degreesC. The CMS membrane films have an intrinsic CO2/CH4 selectivity of 200 with a CO2 permeability of 44 Barrers and an O-2/N-2 selectivity of 13.3 with an O-2 permeability of 24 Barrers at 35degreesC. The pyrolyzed CMS materials were ball-milled into fine particles, ranging in size from submicron to 2 mum, prior to dispersal in casting solvent. Mixed matrix films comprising high CMS particle loadings (up to 35 wt.%) dispersed within two polymer matrices (Matrimido 5218 and Ultem((R)) 1000) were successfully formed from flat-sheet solution casting. For Ultem((R))-CMS mixed matrix membrane films, pure gas permeation tests show enhancements by. as much as 40% in CO2/CH4 selectivity over the intrinsic CO2/CH4 selectivity of the pure Ultem((R)) polymer matrix. Likewise, for Matrimid((R))-CMS mixed matrix films, enhancements by as much as 45% in CO2/CH4 selectivity were observed. Similar enhancements were observed when these mixed matrix membrane films were examined for the O-2/N-2 separation (8 and 20% for the Ultem((R))-CMS and Matrimid((R))-CMS mixed matrix films, respectively). Effective permeabilities of the fast-gas penetrants (O-2 and CO2) through the mixed matrix membranes were also significantly enhanced over the intrinsic permeabilities of the Ultem((R)) and Matrimid((R)) polymer matrices. These encouraging selectivity and permeability enhancements confirm that mixed matrix membrane behavior is achievable with CMS particles.