We observed the surface adsorption layer of propylene, propane and propane/propylene mixed gas on a polyimide film as a function of pressure using X-ray (XR) and neutron (NR) reflectivity. It was initially confirmed by NR that the surface layer is an adsorption layer of propylene using deuterated gas. We have determined by XR the adsorption isotherms of these gases and found that the thickness of the surface adsorption layer of propane and propane/propylene mixed gas is much larger than that of propylene and the isotherm of propane/propylene mixed gas is similar to that of propane. The former and the latter findings predict that the affinity of propane to the polyimide is greater than propylene and the content of propane is much higher than that of propylene in the adsorption layer. We also evaluated the permeability coefficients of these gases as a function of the applied pressure, and found that the permeability coefficients begin to increase at a pressure corresponding to the onset of the adsorption layer. This suggests that the surface adsorption layer of the gas plays an important role in the hydrocarbon gas permeation mechanism in a polymer membrane, while a swelling of the polyimide film is the basis for the increase in the permeability with pressure. (C) 2003 Elsevier Science Ltd. All rights reserved.