TECHNOLOGY FOCUS - ORGANIC CHEMISTRY - Preferred Components: 3C or more hydrocarbon vapor comprises paraffin, olefin, aromatic compound, halogenated compounds, amines, ketones or alcohols. The gas mixture further comprises methane. The selective layer comprises perfluorinated polymer or polymer formed from fluorinated dioxole monomer or fluorinated dioxolane monomer, fluorinated cyclically polymerizable alkylether, or copolymer of fluorinated dioxole and tetrafluoroethylene repeating units. The polymer is represented by formula (I, or II).x,y = relative proportions of dioxole and tetrafluoroethylene blocks, such that x+y = 1;n = positive integer.Preferred Membrane: The separation membrane is integral asymmetric membrane, or composite membrane comprising selective layer supported on microporous support membrane, a gutter layer and a selective layer. The gutter layer comprises copolymer of perfluoro-2,2-dimethyl-1,3-dioxole and tetrafluoroethylene.Preferred Process: The gas mixture containing 5% of 3C or more hydrocarbons which is in contact with the free side of separation membrane, has total 3C or more hydrocarbons partial pressure of 25 psia and pressure of at least 30% of saturation vapor pressure of gas mixture. The post-exposure selectivity of separation membrane is 70% of the pre-exposure selectivity. The separation membrane exhibits a mixed-gas selectivity for hydrogen over methane as measured at operating conditions of process of at least 10, exhibits a mixed-gas selectivity for hydrogen over propane as measured at operating conditions of process of at least 50, and exhibits a mixed-gas selectivity for propylene over propane as measured at operating conditions of process of at least 2.5. SPECIFIC MATERIALS - The desired gas is hydrogen, nitrogen, oxygen, argon, carbon dioxide or propylene.    EXAMPLE - Asymmetric, microporous poly(vinylidene fluoride) (PVDF) support membrane was prepared. The prepared support membrane was dip-coated in 1 weight% of copolymer solution of 40% tetrafluoro ethylene/60% 2,2,4-trifluoro-5-trifluoromethoxy-1,3-dioxole Hyflon AD60 (RTM), in perfluorinated solvent, at 1 ft/min coating speed, and then dried in an oven at 60 degreesC for 10 minutes. The resulting membrane had a selective layer thickness of 0.2-0.5 mum. A sample of finished composite membrane was cut into 12.6 cm2 stamps and tested in a permeation test-cell apparatus with pure gases at 35 degreesC feed temperature and 65 psia feed pressure. During each test, the feed, the permeate, and residue compositions were analyzed by gas chromatography. The gas fluxes (GPU) of the membrane with respect to oxygen, argon, hydrogen and helium were measured, and found to be 180, 85.4, 790 and 1360, respectively. The selectivity (-) of the membrane with respect to oxygen/nitrogen, nitrogen/methane, helium/hydrogen and argon/methane were measured and found to be 3.5, 2.9, 1.7 and 4.8, respectively. The membrane stamps was then tested for its argon/ethylene selectivity and nitrogen/ethylene selectivity, and found to be 7.4 and 4.8, respectively. The carbon dioxide/methane selectivity of the membrane was found to be 17 at 20 degreesC and 115 psia pressure, and found to be 21 at -20 degreesC and 315 psia pressure. The solvent resistance of the membrane with respect to toluene and hexane was compared with polysulfone membrane. The result showed that the polysulfone membrane could not withstand exposure to toluene and its hydrogen/methane selectivity declined by half after exposure to hexane. The Hyflon AD60 (RTM) membrane retained its hydrogen/methane selectivity, even they exhibited higher fluxes for all gases for which they were tested after soaking in methane and toluene.