X-ray reflectivity measurements were performed on a polyimide thin film in propylene. The film was synthesized from 2,2-bis(3,4-carboxyphenyl)hexafluoropropane dianhydride (6FDA). In propylene at a relative vapor pressure P/P-0 > 0.55, an additive layer was detected on the polyimide surface. The layer thickness increased with the relative vapor pressure. At P/P-0 = 0.96, the layer thickness was about 6 nm. However, the thickness decreased rapidly with temperature. It was thought that the layer was composed of propylene. The propylene permeability increased substantially at P/P-0 > 0.6. The adsorbed layer was thought to play an important role in the increase in the propylene permeability.