Perfluorinated, ultrathin polymer films containing sulfonic acid groups were prepared by plasma polymerization of trifluoromethanesulfonic acid and hexafluoropropylene. Electron spectroscopy for chemical analysis and Fourier transform infrared spectroscopy measurements indicated that the sulfonic acid groups were fixed to the backbone chain of the perfluorinated polymer. The structure of the plasma polymer was strongly dependent on the applied RF power. At high levels of applied RF power, the sulfonic acid groups of the starting material were easily decomposed during plasma polymerization. The perfluorinated cation-exchanger films prepared at low applied RF power were about 1 mum thick and free from pinholes. The films showed high cation permselectivity, as confirmed by a K+ ion transference number of 0.99 in aqueous KCl solution.