NOVELTY - A gas mixture comprising 3C or more hydrocarbon (HC) vapor and desired gas, is passed across feedside of separation membrane. Driving force is provided, to withdraw permeate stream enriched in desired gas, and residue stream depleted in desired gas. The membrane has 65% of post-exposure selectivity for desired gas over HC vapor, after exposure of membrane to liquid toluene and subsequent drying.    USE - For separating desired gas like hydrogen from gas mixture comprising hydrogen and 3C or more hydrocarbon vapor, and for separating propylene from gas mixture comprising propylene and propane, used in refineries, petrochemical plants and natural gas fields.    ADVANTAGE - The membrane retains selectivity for desired gases such as hydrogen, nitrogen, carbon dioxide, methane or light olefin, in the presence of streams rich in, or even essentially saturated with, 3C or more hydrocarbons. The membrane has good resistance to plasticization by organic components in the gas mixture under treatment, and good recovery after exposure to liquid aromatic hydrocarbons. The membrane is used for separating hydrogen from methane or 2C or more hydrocarbons or 3C or more hydrocarbons. The membrane maintains useful separation properties in the presence of organic vapors at high activity, and able to recover from accidental exposure to liquid hydrocarbons.    DETAILED DESCRIPTION - The gas mixture comprising 3C or more hydrocarbon vapor and desired gas, is passed across the feedside of separation membrane (3) having feedside and permeate side. A driving force is provided for transmembrane permeation. A permeate stream enriched in the desired gas is withdrawn from the permeate side, and a residue stream depleted in desired gas is withdrawn from feedside. The separation membrane has a selective layer comprising a polymer having fluorine to carbon atom in the ratio greater than 1:1, fractional free volume of 0.3 or less, and glass transition temperature of 100 degreesC. Separation membrane has 65% post-exposure selectivity for desired gas over 3C or more hydrocarbon vapor, after exposure of separation membrane to liquid toluene and subsequent drying, as measured pre- and post-exposure with test gas mixture of same composition and under same conditions.    DESCRIPTION OF DRAWING(S) - The figure shows the schematic representation of separation process.    Separation membrane (3)    Residue stream (4)    Permeate stream (5)