A series of copolyimides containing carboxylic acid groups have been prepared from 2,2-bis(3,4-dicarboxyphenyl) hexafluoropropane dianhydride (6FDA) and two diamines, 2,4,6-trimethyl-1,3-phenylenediamine and bis(4-amino-3-isopropyl-5-methylphenyl)methane. Carboxylic acid groups were introduced using 3,5-diaminobenzoic acid (DABA) as a comonomer in different proportions in both series of polyimides. The polymers were fully characterized using spectroscopic and thermal analysis techniques. The gas permeation properties of the polymer films were investigated and the results were explained mainly by anhydride formation, which depended not only on the -COOH content, flexibility, and structure of the polymer chain but also on the thermal treatment of the membrane. Copolyimides with the lowest DABA content of both series showed the best permeation properties, particularly the copolyimide that contained 25% of acid units, which showed $O_2$, $N_2$, $CH_4$, and $CO_2$ permeability coefficients close to 40, 10, 7, and 145 barrers, respectively. The gas separation properties were evaluated using three different casting solvents, N-methylpyrrolidinone, tetrahydrofuran, and acetone. At the initial stage, large differences in permeability coefficients were obtained for each copolyimide. On the other hand, after 1,000 h aging at room temperature, the membranes of each series showed similar permeability coefficients.