A double-layer liquid membrane was used for the separation of propylene/propane mixed gas. The double-layer membrane was constructed such that a supported liquid membrane consisting of a hydrophilic and non-volatile liquid was supported on the surface of another hydrophobic microporous membrane. This construction enabled the liquid membrane to be used in the permeation mode of vacuum pressure on the permeate side. In the experiment, the feed gas was saturated by water vapor in order to swell the liquid membrane and ionize the silver salt in it. A liquid membrane of a triethylene glycol (TEG) and silver salt, AgBF4, mixture showed selective permeation for propylene. The silver salt acts as facilitating agent of the olefin in the triethylene glycol liquid membrane. Water vapor in the process enhanced the permeation rate and separation factor of the liquid membrane. (C) 2003 Elsevier Science B.V. All rights reserved.