A poly (perfluorosulfonic) ionomer membrane (Nafion N-117) was ion-exchanged with sodium, heated in glycerine and immersed in water to increase the water content from about 15 to about 55 weight %. When a sodium-exchanged membrane was heated dry and quenched prior to the swelling in hot glycerine, the water content was increased to about 67 weight %. Ethene fluxes versus ethene feed pressures were measured for both Na-membranes, and the effective diffusion coefficient for ethene was calculated from flux data. The Na-membranes were soaked in aqueous 2 M AgBF4. The flux of ethene through the Ag-membranes obtained were measured at several feed partial pressures, and the flux data were used to estimate the equilibrium constant for the coordination of ethene to silver ions within the membrane and the effective diffusion coefficients for ethene. Separation of ethene from ethane with the highly water-swollen Nafion Ag-membranes was measured with a 1:1 mole ratio of ethene and ethane as feed at atmospheric pressure. The ethene flux corresponded to a permeability of 9700-12,500 barrer, and the separation factors obtained by the flux ratios were in the range of 500-540. For membranes which were soaked in aqueous 6 M AgBF4, the ethene flux corresponded to a permeability of 26,800 barrer on average for a 1:1 mole ratio of ethene and ethane as feed at atmospheric pressure. The separation factor expressed by the flux ratios was 1950 on average.