A rubbery nylon-12/tetramethylene oxide block copolymer (PA12-PTMO) was used as a polymeric matrix material for silver tetrafluoroborate (AgBF4) based solid polymer electrolyte membranes. Ethane sorption uptake of PA12-PTMO/AgBF4 increased linearly with increasing feed pressure. Moreover, the ethane sorption capacity decreased by increasing the AgBF4 concentration in the polymer electrolyte. Ethylene sorption of pure PA12-PTMO also followed Henry's law. On the other hand, the ethylene sorption isotherms of PA12-PTMO/AgBF4 showed a completely different behavior. As the silver concentration increased in the polymer electrolyte membranes, the sorption isotherms showed a dual-mode sorption behavior. The initial sorption enhancement provides clear evidence of complex formation between ethylene and silver ions. Mixed-gas permeation studies performed with dry ethylene/ethane mixture demonstrated that PA12-PTMO/AgBF4 composite membranes exhibited good stability. In a 14-day test period, the ethylene/ethane selectivity of this membrane decreased from 25-20. This performance is far better than that of any polymeric membrane for ethylene/ethane separation. The decline in membrane performance occurred only during the first 3 days of operation; thereafter, the membrane showed excellent long-term stability.