Perfluorosulfonate ionomer membranes (Nafion) that have been ion-exchanged with silver(I) ion exhibit unexpectedly large selectivities for the separation of certain unsaturated hydrocarbon mixtures, such as styrene from ethylbenzene and linear C-5-C-10 dienes from monoenes. Transmembrane fluxes increase linearly with the reciprocal of membrane thickness with no loss of selectivity for membrane thicknesses between 40 and 2.5 mu m. The large selectivities are due to competitive absorption of the hydrocarbons by the membranes. This effect cannot be predicted from single-component experiments or known complexation constants between the hydrocarbon and aqueous Ag(I). The unexpectedly large separation factors for the diene/monoene mixtures can be explained semiquantitatively by invoking the complexation of dienes to two Ag(I) ions.