Solid polymer electrolyte membranes comprising of poly(ethylene oxide) and silver tetrafluoroborate were prepared for olefin and paraffin permeation with a focus on the study of membrane stability issues. The membrane initially showed very high permselectivities for the permeation of ethylene-ethane and propylene-propane. The high permselectivity, however, could not be retained for a prolonged period of permeation. The membrane stability was investigated by varying the feed pressure and operating temperature in a cyclic fashion. It was observed that in addition to the gradual decline in membrane performance with permeation time that has been observed previously, conditioning of the membrane with the olefin permeant could result in an irreversible decrease in the membrane performance, presumably due to the silver-PEO and silver-olefin complexation interactions. Continued studies on improving the membrane stability are needed for practical olefin-paraffin separation applications. (C) 2003 Elsevier B.V. All rights reserved.