In this study, the morphology. transport properties, selectivity, and direct methanol fuel cell (DMFC) performance of a triblock copolymer ionomer, sulfonated poly(styrene-isobutylene-styrene) (S-SIBS), were investigated. The structure of S-SIBS membranes, ranging from 0.5 to 1.0 meq/g, was characterized using small-angle X-ray scattering and revealed a lamellar morphology with a preferred orientation in the plane of the membrane. Subsequently, proton conductivity measured normal to the plane of the membrane was over 10 times lower than when measured in plane, compared to only a 2.5 times difference in conductivity between the two techniques in Nafion 117. When proton conductivity was measured normal to the plane, the resulting selectivities (i.e. proton conductivity/methanol permeability) of S-SIBS membranes are similar to Nafion 117. A previous study [J. Membrane Sci. 217 (2003) 227] reports 5-10 times higher selectivities compared to Nation 117 when conductivities were measured in plane. In addition, the DMFC performance of S-SIBS (1.0 meq/g) was lower than that of Nation 117. These significant differences in transport properties and fuel cell performance with respect to polymer structure in S-SIBS membranes provide new insights into designing oriented nanostructures in polymer electrolyte membranes for enhanced transport properties for fuel cell applications. (C) 2003 Elsevier B.V. All rights reserved.