Gas permeation experiments of H-2, O-2, CO2, N-2, and CH4 were carried out with freestanding films of the conjugated polymer polyaniline (PANi). At first annealed to remove residual solvent, PANi membranes were doped (i.e., protonated) in a strongly acidic medium (HCl 4M), undoped in a basic medium (NH4OH 1M), and redoped in a slightly acidic medium (KCl 10(-2)M). Protonation and deprotonation kinetics were studied by elementary analysis. Gas permeation experiments were performed with the annealed, doped, undoped, and redoped PANi films. The gas transport mechanism was clearly influenced by the diffusivity factor and it obeyed a Fickian diffusion model. From the variations in permeability coefficients with the doping treatment, gases could be divided in two subgroups comprising H-2, O-2, and CO2 on one hand and N-2 and CH4 on the other. After the doping-undoping-redoping process, gas fluxes were increased by 15% for the smaller gases and were decreased by 45% for the larger gases. As a consequence gas separation factors were approximately doubled for a gas pair involving the two subgroups and these were unchanged for a gas pair involving only one subgroup. The highest O-2/N-2 and CO2/CH4 selectivity coefficients were, respectively, equal to 14 and 78.