A gas permeability apparatus has been constructed to facilitate the study of permeation of mixtures of gases and vapors. The apparatus utilizes the carrier gas method with gas-chromatographic analysis to determine individual permeation values and in-line thermal conductivity detectors to provide simultaneous overall diffusion and permeability data. The effects of film type, structure, and morphology on permeability may be studied from atmospheric to 100 psi feed pressure and from ambient to 300°C. Feed stocks may be custom mixed, and a vaporizing bath permits introduction of vapors either as contaminants or as primary permeants in an inert carrier. The experiments which illustrate the use of the equipment suggest that the noninteracting gases H2 and CH4 permeate a number of polymer films independently over a wide concentration range and that water vapor retards their permeation in polyimide films.