Asymmetric 2,2-bis-(3,4-dicarboxyphenyl) hexafluoropropane dianhydride (6FDA)-2-methyl-1,3-phenylenediamine (mPDA) polyimide membranes were prepared according to a phase-inversion mechanism by using different solvent/nonsolvent blends. The membrane formation mechanism and the final performances of the asymmetric membranes have been found both nonsolvent and solvent nature dependent. From the visualization of cross sections of asymmetric membranes by scanning electron microscopy and the study of the permeation of two gases (N-2, CO2) through asymmetric membranes, a relationship between elaboration conditions and asymmetric membranes characteristics could be drawn. The organization of polymer chains in solution strongly affects the final polymer arrangement and thus the final performances of the membrane. The influence of preliminary solvent evaporation before immersion has been shown to be dependent on the structure of the asymmetric membrane: finger-like or sponge-like structures. (C) 2003 Wiley Periodicals, lnc. J Appl Polym Sci 89: 1838-1848, 2003.