Poly(perfluorosulfonic)ionomer membranes (Nafion N-117) were studied for the separation of ethene from ethane. Membranes in the sodium form were ion-exchanged in an aqueous 2 M solution of Ag+, and the flux of ethene was measured as a function of the partial pressure of ethene in the humidified feed gas. The experimental data were consistent with a carrier transport mechanism for a diffusion limited analYtical model. The plot of the inverse facilitated ethene flux versus the inverse partial feed pressure was used to calculate the equilibrium constant for complexation of silver and ethene within the membrane. Effective diffusion coefficients for ethene were also calculated from the flux data. The facilitation factor for ethene with a humidified feed mixture of 50 mole% ethane and 50 mole% ethene at total atmospheric pressure was 170. The separation factor was 470. The permeability coefficient for ethene corresponded to 1000 barrer.