Three polymers bearing benzene rings : polystyrene, poly(2,6-dimethyl-p-phenylene oxide) and poly(diphenylacetylene), were chemically modified with bis(heptafluorobutyryl) peroxide. The heptafluoropropylated products have higher gas permeabilities than the parent polymers. In the pervaporation of aqueous ethanol, modified poly(diphenylacetylene) showed ethanol permselectivity.