Remarkably high-performance membranes are developed by physically dispersing silver ionic aggregates in inert polymer such as poly(dimethyl siloxane) (PDMS). The mixed gas selectivity for propylene/propane reaches nearly 200, to our knowledge the highest value ever reported for the solid state, whereas other membranes reported up to now show similar to60 of propylene/propane selectivity. Physical dispersion of silver salt in the inert polymers without specific interaction is observed by FT-IR, FT-Raman and wide angle X-ray scattering (WAXS). It is also found that the crystalline silver ionic aggregates are completely dissolved under the propylene environment, representing that propylene is a good ligand for coordination to silver ions. The inert membranes are functional at any loading level and hence a threshold carrier concentration does not exist. (C) 2004 Elsevier B.V. All rights reserved.