Previously we have found that the facilitated olefin transport through polymer/silver complex membranes is sensitive to the type of the counteranion of salt; the separation performances including the permeance and selectivity of propylene/propane are in the order of AgBF4 >AgCF3SO3 AgNO3. Here the polymer/silver salt complex membranes consisting of two mixed salts dissolved in poly(2-ethyl-2oxazoline) (POZ) were prepared and tested for the separation of propylene/propane mixtures. Upon mixing AgNO3 with AgBF4 or AgCF3SO3, the selectivity of propylene/propane unusually increased in certain composition ranges. The permeation behavior was interpreted in terms of both the activity of silver ion as olefin carrier and the structural change of polymer electrolyte membranes using FT-IR spectroscopy and wide angle X-ray scattering (WAXS). As a result, the anomalous enhancement of selectivity was attributable mainly to the structural compactness of membranes, resulting from the increased transient cross-links of polymer chains by coordination of small sized AgNO3. (C) 2004 Elsevier B.V. All rights reserved.