A previous study showed that ionic liquids enable AgNO3, which is not readily reduced to silver nanoparticles in the solid state, to be an active olefin carrier in silver polymer electrolyte membranes. We report the effects of an ionic liquid, i.e. 1-butyl-3-methylimidazolium nitrate (BMIM+NO3-), on the separation properties of polymer/AgNO3 complex membranes for propylene/propane mixtures. Interestingly, the change in membrane separation properties with the ionic liquid concentration was completely different between poly(2-ethyl-2-oxazoline) (POZ) and poly(ethylene oxide) (PEO). In the POZ/AgNO3 complex membranes, both the selectivity of propylene/propane and the mixed gas permeance increased up to a 0.1 mole ratio of ionic liquid, after which they decreased. The enhancement of the membrane performance results from an increase in the silver ion activity, whereas the decrease was attributed to the barrier effect of large amounts of IL and less effectiveness of IL in controlling the ionic interactions. On the other hand, in the PEO/AgNO3 membranes, the selectivity increased with increasing ionic liquid, whereas the mixed gas permeance decreased. The increase in selectivity comes from the enhanced silver ion activity, whereas the decrease in permeance was attributed to the decrease in interfacial voids in the membranes.