End-group cross-linkable sulfonated poly(arylene ether) polymer (E-SFQK) was synthesized via direct polymerization of potassium 2,5-dihydroxybenzenesulfonate (SHQ) and decafluorobiphenyl (DFBP), followed by a reaction with ethynylphenol (EP). The cross-linking reaction of the ethynyl end group of E-SFQK was performed at 250 degrees C. After cross-linking, proton conductivity, water uptake, and swelling ratio of cross-linked membrane decreased from 0.16 (noncross-linked membrane) to 0.13 S/cm. from 86% to 42%, and from 31% to 13%, respectively. The effect of cross-linking time on proton conductivity, water uptake, and swelling ratio were also investigated. Methanol permeability of cross-linked membrane was compared with Nafion 117 due to solubility of noncross-linked membranes in methanol. The cross-linked membrane performed better, with a methanol permeability of 88 x 10(-8) cm(2)/s, as compared with 154 x 10(-8) cm(2)/S for Nafion 117. The cross-linked membrane also exhibited improved chemical resistance and oxidative stability from solubility and Fenton's tests. In order to study morphological changes of cross-linked and noncross-linked membranes, hydrophilic domain sizes from an AFM phase image were evaluated. These results showed that the sizes of hydrophilic domains of the crosslinked membrane (5-20 nm) are much smaller than those of noncross-linked membrane (20-50 nm).