a pervaporation process for separating organic liquid mixtures of components having differing polarities , uses an ion exchange membrane which comprises a polymerized alkene including pendant groups of formula so 2 n — a — qx where r is h or alkyl , a is alkylene , q is quaternary ammonium , and x is a negatively - charged counter - ion , the sulfur content of the membrane being in the range 3 - 10 wt . %.