We have investigated the water-transport properties of composite membranes prepared by coating a thin film of a sulfonated polystyrene across the surface of a microporous alumina support membrane. Such composite membranes are designated PSS/Al2O3 composites in this paper. The transport data were obtained by exposing the PSS face of the PSS/Al2O3 composite to a vapor phase of known water partial pressure. Water was sorbed by the PSS and transported across the composite membrane to a stream of dry argon. The concentration of water in the permeate stream (as a result of transport of water across the membrane) was measured using a dew-point measuring device. A mathematical model (used in a previous investigation of water transport in the sulfonated perfluorocarbon polymer Nafion(r)) was used to analyze the transport data for the PSS/Al2O3 composite membranes. These investigations showed that the solubility of water in the PSS film increased with water vapor pressure in the feed stream. The permeability coefficient for water in the PSS film also increased with water vapor pressure in the feed stream. In contrast, diffusion coefficients for water decreased with vapor pressure in the feed stream, and hence with water content of the PSS film. The effect of sulfonation level and of the nature of the counterion on the magnitude of the water diffusion coefficient was also investigated.