A Fourier transform infrared/attenuated total reflectance technique was used to study the diffusion of water through poly(styrene-b-isobutylene-b-styrene) block copolymers (BCPs), as well as sulfonated (H+) and Na+-sulfonated ionomer versions. Diffusion data were collected and interpreted for these membranes versus polystyrene block composition, degree of sulfonation, Na+ ion content in the ionomers, and the effect of initially dry versus prehydrated conditions. An "early time" diffusion coefficient, D, decreased with increasing percent polystyrene for a series of unmodified BCPs. D decreased with increasing degree of sulfanation, and with increasing ion content for the Na+-exchanged samples and this was interpreted in terms of diffusion limitations caused by a strong tendency for ion hydration. The method also yielded information relating to the time evolution of water structure from the standpoint of degree of intermolecular hydrogen bonding. Membrane prehydration causes profound increases in D for both the unmodified BCP and sulfonated samples, as in plasticization. The simultaneous acquisition of information relating to interactions between water molecules and interactions of water molecules with functional groups on the host polymer matrix offers more information than conventional diffusion measurement techniques that simply count transported molecules. (c) 2005 Wiley Periodicals, Inc.