Integrally skinned asymmetric polysulfone membranes were prepared from originally dense films inducing asymmetry by the formation of the porous layer adding to one side of the membranes chloroform and supercritical CO2 (SCCO2), and then allowing the SCCO2 expansion to occur. The influence of the chloroform/polysulfone mass ratio (g CH3Cl/g PSF), SCCO2 density and depressurization rate over the thickness of both the porous and the dense skin layers, the morphology of the porous support and the pure O-2 and N-2 permeability and selectivity performance were studied. The results show that it is possible to induce a very-controlled asymmetry in a dense film following the procedure described in this work and as expected, the thickness of the porous layer increases while the dense skin layer decreases as the chloroform/polysulfone mass ratio increases. Images of the porous layer show that the average-pore size decreases at high SCCO2 densities and slightly decreases with increasing the CO2 depressurization rates. The O-2 and N-2 permeability coefficients, measured at 35 degrees C and 2 bar, for the polysulfone asymmetric membranes are practically the same of those determined in dense films, suggesting that the dense skins are essentially defect-free of pinholes. (C) 2008 Elsevier B.V. All rights reserved.