Designed room temperature ionic liquids (RTILs) containing silver salt are presented as reactive media in separating propylene/propane gas mixtures. Solubilities of propylene and propane in the reactive media, silver tetrafluoroborate (AgBF4) dissolved in 1-butyl-3-methylimidazolium tetrafluoroborate (BmimBF(4)) and N-butyl-4-methylpyridinium tetrafluoroborate (BmpyBF(4)), were investigated as a function of silver ion concentration, temperature, and pressure. Equilibrium data were obtained working in a temperature range between 278 and 318 K and at pressures up to 6 bar. Propylene absorption was chemically enhanced in the silver-based RTILs and was considerably higher than that in the standard RTILs. Absorption of propane in the silver-based RTILs is based on physical interactions only. A simple mathematical model based on the formation of complex species with different stoichiometry has been developed in order to describe the total propylene absorption, and the model was validated with experimental data obtained working with different concentrations of silver salt (between 0.1 and 1 M). The model parameters, equilibrium constants (K-Eq,K-1 f(T) and K-Eq,K-2 f(T)), and enthalpies of complexation (Delta H-r,H-1, Delta H-r,H-2) were obtained. Thermal stability of the silver ions was analyzed and to be found dependent on the silver salt concentration. Complete regeneration of the reaction media was possible at a temperature of 313 K and 20 mbar of pressure.