Ethylene and propylene are produced in larger quantities than any other organic compound. Production of these olefins requires separation of the olefins from the corresponding paraffins. Distillation is currently used but this is an extremely energy-intensive process due to the very low relative volatility of the components. Previous studies have shown that facilitated transport membranes can have high selectivity for olefin/paraffin separation. However, four problems have limited the commercial application of facilitated transport membranes: (i) poor mechanical stability, (ii) the difficulty in preparing thin, high-flux composite membranes, (iii) the requirement of a water-vapor-saturated feed to provide mobility for the olefin-selective carrier, and (iv) poor chemical stability due to carrier poisoning. Solid polymer electrolytes are a novel class of facilitated transport membranes for olefin/paraffin separation. These membranes solve the first three problems listed above. Solid polymer electrolyte membranes are based on rubbery, polyether-based polymers containing a dissolved olefin-complexing metal salt. Solid polymer electrolyte composite membranes made from poly(ethylene oxide) loaded with silver tetrafluoroborate showed an ethylene/ethane selectivity of up to 240 and an ethylene permeance of 8 x 10(-6) cm(3)(STP)/cm(2) s cmHg with a dry feed gas mixture.