Asymmetric membranes with a dense skin were prepared by the dry phase inversion technique, from a casting solution of a modified poly(ether ether ketone), PEEKWC, in dichloromethane, chloroform and tetrahydrofuran as solvents and C-1-C-4, C-8 aliphatic alcohols and/or water as nonsolvents. The phase diagram of the different polymer/solvent/nonsolvent combinations was determined by cloud point measurements. The influence of the polymer concentration, type of solvent, type and concentration of nonsolvents on the morphology, observed by scanning electron microscopy, and on the transport properties was studied. The obtained results were correlated with the ternary phase diagrams and were explained in terms of thermodynamics and kinetics of the phase inversion process. The membrane preparation conditions were optimized in order to reduce the skin thickness and increase the gas fluxes consequently. Low nonsolvent volatility was found to be more important than a high solvent hardness for obtaining a high void fraction and a thin dense skin. Thus, from PEEKWC solutions in chloroform, with butanol as the nonsolvent, membranes with a dense skin of about 1 μ m were obtained, exhibiting a CO2/N-2 selectivity and an O-2/N-2 selectivity of 33 and 6, respectively, comparable to typical commercial membranes, but at a slightly lower permeance (8.3 x 10(-8) m(3)/(m(2) h Pa) (3.1 GPU) for CO2 and 1.5 x 10(-8) m(3)/(m(2) h Pa) (0.56 GPU) for O-2) than typical commercial membranes, like polyimide composites. © 2005 Elsevier B.V. All rights reserved.