A series of sulfonated poly(ether sulfone) (SPES) were prepared by sulfonating poly(ether sulfone) with chlorosulfonic as sulfanating agent and concentrated sulfuric acid as solvent. Tough and ductile SPES membranes were obtained from DMAc solution. The degree of sulfonation (DS) and ion exchange capacity (IEC) were calculated via H-1 NMR and titration. Microphase separated structure comprised of hydrophilic ionic clusters and hydrophobic polymer backbone were observed from atomic force microscopy (AFM) phase images. The hydrophilic ionic clusters became continuous to form channels when DS reached 0.50, corresponding to suddenly increase of water uptake. Combining AFM with water uptake analysis, we found SPES system reached a percolation threshold around DS of 0.39. SPES membranes with DS below 0.39 showed good dimensional stability in water, mechanical stability and oxidative stability. Proton conductivities ranged from 10(-4) to 10(-1) S/cm were measured at room temperature in water. SPES membranes exhibited lower methanol permeability than Nation 112. A highest value of the proton conductivity-to-methanol permeability ratio was obtained for SPES with DS of 0.39. The results showed that SPES was promising for possible use in proton exchange membrane fuel cells, especially in direct methanol fuel cells. (c) 2006 Elsevier B.V All rights reserved.