Asymmetric gas separation membranes were prepared by the dry-casting technique from PEEKWC a modified amorphous glassy poly(ether ether ketone). The phase inversion process and membrane performance were correlated to the properties of the polymer and the casting solution (molar mass, polymer concentration, solution rheology and thermodynamics). It was found that I broad molar mass distribution of the polymer in the casting solution is most favourable for the formation of a highly selective membrane with a dense skin and a porous sub-layer. Thus, membranes with an effective skin thickness of less than I mu m were obtained, exhibiting a maximum O-2/N-2 selectivity of 7.2 and a CO2/CH4 selectivity of 39, both significantly higher than in a corresponding thick dense PEEKWC membrane and also comparable to or higher than that of the most commonly used polymers for gas separation membranes. The CO2 and O-2- permeance were LIP to 9.5 X 10(-3) and 1.8 X 10(-3) m(3)/(m(2) h bar) (3.5 and 0.67 GPU), respectively. (c) 2005 Elsevier Ltd. All rights reserved.