Ultrathin, uniform, pinhole-free films containing sulfonic ester groups were prepared by plasma-polymerization of 1,3-butadiene and methyl benzenesulfonate. The sulfonic ester groups of the resulting plasma polymer were transformed to lithium sulfonate by reaction with lithium iodide. The sulfonate cation-exchange films thus prepared had excellent permselectivity, as shown by the K+ ion transference number of 0.99 in aqueous KCl, and a low resistance per unit area, of 0.04 OMEGA-cm2, in 0.5M aqueous H2SO4 (ionic conductivity = 1.8 x 10(-4) S cm-1).