Composite membranes with ultrathin separation layer were prepared by alternate adsorption of cationic and anionic polyelectrolytes onto porous support membranes. The utility of these membranes for ethanol-water pervaporation was investigated. It was found that the separation capability is highest if polyelectrolytes with high charge density, such as polyethyleneimine (PEI) and polyvinylsulfate (PVS), are used. Under optimum conditions, the PEI/PVS separation layer is highly efficient in water enrichment in the permeate. A separation factor alpha up to 1000 could be detected, the flux simultaneously being 590 g/m(2)h. Effects of annealing of the membrane and change of the pervaporation temperature are also discussed.