This paper describes the performance of sulfonated aromatic poly(ether ether ketone) (S-PEEK) membranes, directly prepared from the sulfonated monomer, for CO2 separation from gas mixtures containing N-2 or CH4. Dense membranes with different degrees of sulfonation were prepared via solvent evaporation. Increasing degree of sulfonation simultaneously improves the permeability and selectivity of both gas pairs. The effect of counterions was investigated by converting S-PEEK membranes from the Na+-form in which they are prepared, to the H+ and multivalent cationic forms. Gas permeability and selectivity for polymers with divalent and trivalent counterions were higher than those for polymers in the monovalent and H+-forms. In order to study the stability and potential industrial application of these membranes, they were tested at different conditions of feed pressure, temperature and CO2 feed concentration. (C) 2011 Elsevier By. All rights reserved.