Polydimethylsiloxane (PDMS) is the most commonly used membrane material for the separation of organic vapors from permanent gases. Gas permeabilities of unfilled, isotropic PDMS films were determined over a temperature range of 35 to -20degreesC for pure gases and a series of binary n-butane/methane and multicomponent hydrocarbon/hydrogen mixtures. The permeabilities of all gas mixture components depend strongly on feed gas composition and temperature. An increase in organic vapor concentration, at constant feed pressure and temperature, leads to increased chain mobility in PDMS, and, therefore, higher diffusivity and permeability. The relative increase in permeability is greater for large, condensable vapors, such as n-butane, than for small, permanent gases, such as hydrogen or methane. As a result, the mixed-gas hydrocarbon/methane and hydrocarbon/hydrogen selectivities increase by increasing the feed vapor concentration. A decrease in feed temperature also results in higher organic vapor permeability and organic-vapor/permanent-gas selectivity. The mixed-gas n-butane/methane selectivity of a PDMS film increased from 9 at 35degreesC to 25 at -20degreesC using a binary feed mixture containing 2 vol.% n-butane in methane. (C) 2004 Elsevier B.V. All rights reserved.