Inert composite membranes, in which silver salts are physically dispersed in rubbery poly(dimethyl siloxane) (PDMS), were prepared and tested for the separation of propylene/propane mixtures. Physical dispersion of silver salts in the PDMS matrix without specific interaction leads to very low separation performances initially. However, as the permeation time increases, both the selectivity and the permeance increase continuously to finally reach equilibrium values. These unusual permeation properties are likely to be due to the conversion of the initially inactive silver ionic aggregates into the more active free ions through the coordination of propylene to the silver ions under a propylene environment. Physical dispersion of the salts and the dissolution behavior by propylene were investigated using FT-IR and FT-Raman spectroscopy and wide-angle X-ray scattering (WAXS). The separation performance of the membrane for propylene/propane mixtures is to our knowledge the highest ever reported for the solid state; the mixed gas selectivity was 200.