Asymmetric 2,2-bis (3,4-dicarboxyphenyl) hexafluoropropane dianhydride (6FDA)-2-methyl-1, 3-phenylenediamine (mPDA) polyimide membranes were prepared by phase inversion process. Using different non-solvents and solvents, the correlation between membrane elaboration conditions acid final morphology and properties has been investigated. Equilibrium thermodynamics and diffusion coefficients data were determined as a preliminary analysis in order to underline competitivity between thermodynamic and kinetic parameters. Scanning electron microscopy and gas permeation experiments (CO2, N-2) were used for the determination of structure-permeability relationships. Sponge-like structure membranes and highest selectivity were obtained when using non-solvent/solvent/6FDA-mPDA systems having important coagulation time.