Transport rates (permeability) and ideal separation factors for several gas pairs through dense polyaniline membranes are reported. The ideal separation factors for all gas pairs tested were found to be independent of the polyaniline membrane thickness whereas the permeability of the single gases showed significant variations. Both dedoped and redoped films (film thickness between 9 and 67 mum) were studied. The highest selectivities alpha ((A/B)) found were 7.6 for the gas pair H-2/CO2 in the case of the dedoped membrane and 10 for the gas pair H-2/CO2, 6 for O-2/N-2 and 200 for H-2/N-2 in the case of the redoped membrane. Statistical analysis of a large number of membranes allowed the critical comparison with results obtained by other groups. Comparison with other membrane materials shows that an approximately sixfold enhancement of the respective separation factors is possible for gas pairs containing hydrogen. Similar separation factors are observed for the gas pairs CO2/O-2, CO2/N-2 and N-2/O-2. Membranes for which Knudsen diffusion was observed exhibited regularly distributed micropores (400 nm diameter).