A method for the sulfonation of PEEK-WC, a glassy poly(ether ether ketone) with sulphuric acid is presented. Depending on the reaction time, polymers with ion exchange capacity (IEC) from 0.30 to 0.76 meq(H+)/g are obtained, as determined by titration with NaOH solutions. The thermal properties of the polymers were studied by differential scanning calorimetry, showing that the glass transition temperature increases with increasing degree of sulfonation, from 224 degrees C for pure PEEK-WC to 246 degrees C for the polymer having an IEC of 0.76 meq(H+)/g. The sulfonated polymers were used to prepare proton exchange membranes for possible application in fuel cells. Dense membranes were prepared by solvent evaporation, using DNM as the solvent. The transport properties of the membranes were determined in terms of water uptake and permeability for hydrogen and oxygen. Electrochemical characterization was performed by measuring cell voltage and power density curves as a function of current density at different working temperatures and the results were compared with those of a, commercial Nafion membrane. A power density of 284 mW/cm(2) was obtained for S-PEEK-WC membrane at 120 degrees C in H-2/air fuel cell, slightly above the corresponding value found for Nafion. (c) 2005 Elsevier B.V. All rights reserved.