Abstract Gas permeability values are reported for films of the conducting polymer polyaniline in the emeraldine oxidation state. Substantial improvements in gas selectivity, when compared to literature values, were observed for all gas pairs studied (He/N 2, H 2/N 2, O 2/N 2, CO 2/CH 4). The doping procedure which enables conductivity in the film was found to cause significant concurrent changes in the film morphology which resulted in improved gas separations. These morphological changes primarily influence the permeant diffusivity as shown by comparison to the kinetic diameter of the permeant. The potential for development of conducting polymer membranes with precisely controlled morphology is clearly indicated, allowing optimization of films for any gas separation.