Pure gas permeabilities of propylene and propane through ionic liquids supported on porous alumina membranes were determined as a function of silver salt concentration from 0 to 1 M and feed pressure from 0.15 to 2.0 bar at 35 degrees C. Two ionic liquids were tested: 1-hexyl-2,3-dimethylimidazolium bis-(trifluoromethylsulfonyl)imide ([hmmim][Tf2N]) and 1-hexyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([hmim]-[Tf2N]). Propylene permeability increased with increasing silver content due to facilitated transport, but it decreased with increasing feed pressure. Permeability selectivity exhibited a nonlinear upward trend with silver concentration, indicating the formation of higher order silver-propylene complexes at higher silver concentrations. Solubility measurements were performed gravimetrically, which allowed for simultaneous determination of diffusion coefficients. Permeabilities were hindered by low diffusion coefficients due to increasing viscosity with increasing silver content. The most surprising result was the stability of the membrane after exposure to hydrogen gas, suggesting that the ionic liquids significantly slowed silver ion reduction.