Membrane gas separations are attractive because of their simplicity and low energy costs, but often limited by insufficient gas flux. This problem is especially challenging because the permeability of a material is frequently inversely related to its selectivity. In this paper a novel method to make reproducible defect free self-supported polyaniline films with thicknesses between 2 and 6 mu m, and polyaniline nano-membranes with a selective layer thickness as thin as 300 nm supported on a porous polyvinylidene difluoride (PVDF) structure is described. Their structure and separation performance have been analysed. The permeability, gas transport rates and ideal separation factors for several gas pairs were calculated from experimental data. For dense, polyaniline membranes, ideal separation factors for all gas pairs tested were independent of the membrane thickness. The selectivities, alpha(A/B) (ideal), for the gas pairs H-2/N-2 (348), H-2/O-2 (69.5), H-2/CO2 (8.6), CO2/O-2 (8.1), CO2/N-2 (40.4) and O-2/N-2 (7.1), achieved for self-supported undoped polyaniline films are considerably higher than those obtained by other researchers. Ideal separation factors for polyaniline nano-membranes were similar to those obtained for self-supported polyaniline films. Gas transport rates for various gases, observed for the dense polyaniline nano-films supported on polyvinylidene difluoride were of the order of 10(5) times higher than those reported for self-supported polyaniline membranes. (c) 2006 Elsevier B.V. All rights reserved.