Solid-state silver polymer electrolyte membranes containing AgNO3 dissolved in poly(2-ethyl-2-oxazoline) (POZ) as an olefin carrier are not efficient for the separation of olefin/paraffin mixtures (selectivity similar to 1). The addition of fumed silica nanoparticles into the POZ/AgNO3 complex membranes, however, resulted in a surprisingly significant enhancement of the permeance and selectivity (higher than 80) for propylene/propane mixtures. These enhancements are explained in terms of the silver ion activity in olefin coordination. FT-IR, X-ray photoelectron spectroscopy (XPS) and FT Raman demonstrate that the silver ion activity continuously increases by Lewis acid-base interactions upon incorporation of the silica nanoparticles up to 0.1 mole ratio of silica, above which it abruptly decreases. Furthermore, the structural change of polymer electrolyte membranes took place by the presence of nanoparticles, supported by the Tg measurement. (c) 2006 Elsevier B.V All rights reserved.