The SYMPLEX composite membrane is based on the complexing reaction of poly-ions. The reaction of polyanion and polycation gives a water insoluble membrane with low mechanical stability. In order to improve the mechanical behaviour, composite membranes were prepared in situ on a microporous, chemical resistant support. These membranes have been investigated for the dehydration of organic compounds by pervaporation. SYMPLEX composite membranes are well Suited to dehydration processes by pervaporation of higher alcohols and other organic compounds. Selectivities up to 20 000 and a water Bur of 1.6 kg/h m(2) for the dehydration of alcohols at water feed concentrations above 2 wt% could be obtained. Below these concentrations a decrease in selectivity could be observed, if n-alcohols such as methanol or ethanol which are very similar to water, are to be dehydrated. With other organic compounds such as ketones or amines, a high selectivity and flux could also be detected. High concentrations of strong bases in the feed destroyed the selective layer and in some cases the support, too.