The transport properties of He, H-2, CO2, O-2, N-2, and CH4 gases in solvent cast, HCl doped, and undoped polyaniline (PANi) membranes were determined. Measurements were carried out at 40 psi pressure from 19 degrees C to 60 degrees C. An excellent correlation was found between the diffusion coefficients and the molecular diameters of gases. The solubility coefficients of gases were found to correlate with their boiling points or critical temperatures. The separation factors for CO2/N-2 and CO2/CH4 are dominated by the high solubility of CO2. These correlations enable us to predict the permeability, diffusion, and solubility coefficients of other gases. After the doping-undoping process, the fluxes of gases with kinetic diameters smaller than 3.5 Angstrom increased but those of larger gases decreased. This results in a higher separation factor for a gas pair involving a small gas molecule and a larger one.