A series of cross-linkable poly(arylene ether ketone)s (PAEKs) with sulfonic acid groups on both the backbone and pendant positions are synthesized through direct condensation polymerization. The degree of sulfonation (DS) of the polymer backbone is controlled by changing the feed ratios of sulfonated to unsulfonated monomers. Post-polymerization reactions are successfully employed to prepare polymer electrolyte membranes for fuel cell applications. The chemical structures of the synthesized polymers are confirmed by H-1 nuclear magnetic resonance (NMR) and Fourier transform infrared (FT-IR) spectroscopy. The thermal and mechanical properties of the membranes are characterized by thermogravimetric analysis and stress strain test. The dependence of water uptake, methanol permeability, proton conductivity, and selectivity on DS is studied. The cross-linked sulfonated PAEK (CSPAEK) membranes with DS higher than 120% exhibit higher proton conductivity than those of Nafion (R) 117. The CSPAEK membranes maintain very low methanol permeability and water uptake. The properties of all the membrane are correlated with ionic cluster dimension measured by small angle X-ray scattering (SAXS). (C) 2012 Elsevier B.V All rights reserved.