Investigations of the rate and selectivity of gas transport in the electronically conductive polymer polyaniline are described. Both rate and selectivity were found to be dependent on the doping level. Permeability coefficients for all gases studied decreased with increasing doping level, and selectivity coefficients, in general, increased with doping level. The effects of other variables, including the nature of the counterion, temperature and duration of thermal processing of the film, were also explored. Both free-standing polyaniline films (approximately 25-mum thick) and thin film composite membranes based on polyaniline were studied. The thin film composites were prepared by coating the polyaniline film (approximately 3-mum thick) onto the surface of a microporous alumina support membrane. The free-standing and thin film composite membranes showed identical rates and selectivities of gas transport. The highest O2/N2 and CO2/CH4 selectivity coefficients obtained were alpha(O2/N2) = 15 and alpha(CO2/CH4) = 55, respectively.