Perfluorinated thin cation-exchanger films fixed with sulfonic acid group were prepared by plasma polymerization of trifluoromethanesulfonic acid and hexafluoropropylene. The influence of polymerization pressure on their structural changes was focused. FT-IR and ESCA measurements indicated that the decomposition of sulfonic acid group was remarkable under low polymerization pressure conditions. The higher the pressure, the more the decomposition was suppressed and the more the sulfonic acid group was introduced. The plasma polymer formed at polymerization pressure as high as 0.5 Torr was about 1 mum thick and free from pinhole, and gave an ionic conductivity of 1.8 X 10(-4) S cm-1 and a K+ transference number was as big as 0.99 in an aqueous KCl solution showing a high cation perm-selectivity.