Gas permeation experiments of O-2 and N-2 were performed with conducting polyaniline (PANi) composite membranes prepared by using a porous nylon membrane as a support. PANi composite membranes were easily obtained by a novel solvent welding process. Doping, dedoping, and redoping kinetics of PANi composite membranes were studied by calculating the [Cl]/[N] content using elemental analysis. After doping and dedoping processes, the permeability of a dedoped PANi membrane decreased while selectivity slightly increased, probably because of the changes in morphology of PANi. d spacing of the PANi film decreased from 4.89 to 3.67 Angstrom after doping, dedoping, and redoping. As redoping continued, the degree of doping increased while the d spacing decreased, resulting in a dramatic increase in selectivity of a PANi membrane. The highest O-2/N-2 selectivity and permeability we obtained from PANi redoped for 2 h were 28 and 0.13 barrer, respectively. O-2/N-2 permeation experiments with different upstream pressure and temperature were carried out to study the permeation behavior of a PANi membrane.