Polymer electrolyte membranes consisting of AgBF4 dissolved in poly(ethylene phthalate) (PEP) exhibit significantly better long-term stability of performance in the separation of propylene/propane mixtures than AgBF4 in poly (2-ethyl-2-oxazoline) (POZ) membranes. This improvement in stability is due to the suppression of the reduction of silver ions to silver metal nanoparticles by the formation of stable chelates between the phthalic groups of PEP and silver ions, which was confirmed by the observation of a large shift of the PEP C=O stretching band with the addition of AgBF4; the peak shift for PEP/AgBF4 is twice that for POZ/AgBF4. Transmission electron micrograph (TEM) results confirmed that the rate of silver metal nanoparticle formation in PEP/AgBF4 is significantly lower than that in POZ/AgBF4. We conclude that the strong coordination between phthalic groups in PEP and silver ions which results in the formation of stable chelates is of pivotal importance in preventing the reduction of silver ions to silver metal and consequently in improving the performance of the membrane in the separation of propylene/propane mixtures. (C) 2004 Elsevier B.V. All rights reserved.