Crosslinkable sulfonated poly(arylene ether ketone) copolymers with various degrees of sulfonation (DS) were prepared and evaluated for use as proton exchange membranes (PEM). The copolymers were synthesized via step growth polymerization using 2.8-dihydroxynaphthalene-6-sulfonated salt (2,8-DHNS-6), 4,4'-(hexafluoroisopropylidene) diphenol (6F-BPA), 4,4'-biphenol (BP), and 4,4'-difluorobenzophenone (DFBP). H-1 NMR and FTIR-ATR spectra confirmed the structure of the copolymers and were also used to determine the obtained DS values of the copolymers. Crosslinked membranes were obtained by thermal curing of the crosslinkable copolymers. The membranes formed a crosslinking network structure via covalent bonding. The glass transition temperatures (T-g) of the crosslinked membranes were determined by differential scanning calorimetry (DSC) and ranged from 208 to 258 degrees C. The decomposition temperatures (T-d) of the membranes ranged from 309 to 325 degrees C for acid forms. The resulting membranes were characterized via an evaluation of their essential membrane properties, such as ion exchange capacity (IEC), water uptake, and proton conductivity. The performance of the membranes was found to be dependent on the DS and IEC. (C) 2009 Elsevier B.V. All rights reserved.