A diblock copolymer ionomer containing a rubbery poly(dimethylsitoxane) block has been developed as a proton exchange membrane for direct methanol fuel cell (DMFC). The partially sulfonated polystyrene-b-poly(dimethylsitoxane) (sPS-b-PDMS) membrane with 38% sulfonation degree exhibited 3 times lower methanol permeability and 2.6 times higher membrane selectivity (proton conductivity/methanol permeability) compared to Nafion (R) 115 at 25 degrees C. Coexistence of microphase domains and ionic clusters was confirmed from the morphological studies by small-angle X-ray scattering and tapping-mode atomic force microscopy. Gas chromatographic analysis revealed that water/methanol selectivity of sPS-b-PDMS was 20 times higher than that of Nafion (R) 115. Such a high water/methanol selectivity can be attributed to the existence of PDMS microdomains minimizing methanol permeation through hydrophilic ion channels. sPS-b-PDMS membranes were fabricated into membrane electrode assembly (MEA), and air-breathing DMFC test for these MEAs showed a better performance compared to the MEA composed of Nafion (R) 115. (C) 2008 Elsevier B.V. All rights reserved.