Hydrolytically stable, proton conductive membranes were prepared from cross-linked poly(arylene ether ketone). The reduction of proton conduction caused by cross-linking was compensated for using a proton conductive cross-linker, 2,2'-benzidinedisulfonic acid. Sulfonation was achieved at pendant positions using sulfanilic acid (4-aminobenzenesulfonic acid) as its acidity is greater at pendant positions than at aryl positions. Fully sulfonated membranes were prepared with up to 30% cross-linking. Despite the high degree of sulfonation, the cross-linked membranes showed much lower water uptake than Nafion(R). The membranes' low methanol permeability, ca. 20%-25% that of Nafion(R), led to selectivity, the ratio of proton conductivity to methanol permeability, about 3 to 4 times that of Nafion(R), despite showing slightly lower proton conductivities. Small angle X-ray scattering showed ionic clusters of around 3.15 nm, which was slightly decreased by cross-linking. This decrease lessened all the investigated membrane properties, water uptake, proton conductivity, and methanol permeability. (C) 2012 Elsevier B.V. All rights reserved.