Multi-block copolymers of sulfonated poly(aryl ether ketone) (PAEK) were synthesized by polycondensation of aromatic dihalide, bisphenol and hydroxy-terminated sulfonated oligomer, which was synthesized from sulfonated aromatic dihalide and bisphenol. These sulfonated PAEKs had benzophenone units and methyl moieties in their structure, and they could be cross-linked by UV irradiation. The effects of the multi-block structure were investigated by comparison of the properties of the multi-block copolymer and a random copolymer. The multi-block structure could reduce the methanol permeability efficiency without decreasing the proton conductivity, and it was concluded that the combination of a multi-block structure and a cross-linking system is a useful method for sulfortated PAEKs for the proton-exchange membrane in a DMFC. (c) 2007 Elsevier B.V. All rights reserved.