Novel 4,4'-dihydroxy-alpha-methylstilbene (HMS)-based sulfonated poly(arylene ether sulfone) with sulfonic acid composition ranging from 10 to 40 mol% was synthesized via nucleophilic step polymerization of 4,4'-dihydroxy-alpha-methylstilbene, 4,4'-dichloro-3,3'-disulfonic acid diphenylsulfore, and 4,4'-dichlorodiphenylsulfone and blended with silica sol to form organic/inorganic nano-composite membranes. The organic/inorganic nano-composite copolymers produced show a high glass transition temperature and thermal decomposition temperatures from 318 to 451 degrees C. The copolymers present appropriate toughness during the membrane process. The equilibrium water uptake and proton conductivity of the obtained organic/inorganic nano-composite membranes were measured as functions of temperature, degree of sulfonation and silica content. In general, the water uptake increased from 8 to 37 wt.%, and the proton conductivity of the organic/inorganic nano-composite membranes increased from 0.003 to 0.110 S cm(-1) as the degree of sulfonation increased from 10 to 40 mol%, the silica content increased from 3 to 10 wt.%, and the temperature increased from 30 to 80 degrees C. The single cell performance of the 40 mol% organic/inorganic nano-composite membrane with various silica contents ranged from 11 to 13 mW cm(-2) at 80 degrees C, and the power density was higher than Nafion((R)) 117. Including the thermal properties, swelling, conductivity and single cell performance, the nano-composite membranes are able to satisfy the requirements of proton exchange membranes for direct methanol fuel cells (DMFC). (c) 2007 Elsevier B.V. All rights reserved.