Most polymeric solvent dehydration pervaporation membranes are made from cross-linked hydrophilic polymers such as cellulose esters or polyvinyl alcohol. At low feed water concentrations, these membranes have very good water/ethanol selectivities. However, the membranes absorb water and swell significantly at higher feed water concentrations, leading to a significant loss of selectivity. We have found that coating these hydrophilic membranes with a thin hydrophobic perfluoropolymer layer can effectively prevent the swelling of the underlying hydrophilic membranes. These coated membranes display a higher selectivity than either the hydrophilic base membrane or the perfluoro coating material alone, when measured with the same feed solution. This counterintuitive result is explained by analyzing the transport behavior of the coated membrane.