In this study, the transport properties (i.e. methanol and proton transport) of a triblock copolymer ionomer, sulfonated poly(styrene-isobutylene-styrene) (S-SIBS), at various ion contents were investigated for its application to the direct methanol fuel cell (DMFC). Methanol permeabilities in S-SIBS were more than an order of magnitude lower than Nation 117, but conductivities were only three-fold less at ambient conditions. S-SIBS was approximately 5-10 times more selective (i.e. proton conductivity/methanol permeability) than Nation 117 over an ion content range of 0.5-1.0 mmol/g. Also, the transport results follow a power law dependent percolation model, where the critical exponent for diffusion was much lower than those reported in other studies, suggesting a more ordered nanostructure. In addition, transport at higher temperatures (80 degreesC) was examined. At 80 degreesC, S-SIBS was only 1.3 times more selective than Nafion 117 at an ion content of 1.0 mmol/g, but was 39 times more selective at an ion content of 0.5 mmol/g. (C) 2003 Elsevier Science B.V. All rights reserved.