The sorption and diffusion of dichloromethane liquid in a wide range of activities and at different temperatures in poly(ether imide) (PEI) films was studied. They were prepared by solution casting at slow solvent evaporation conditions. The sorption behavior is characterized by two zones depending on temperature and penetrant activity. In zone I, at low activity, the isotherms follow Henry's law. The range of activity between 0.25 and 0.30 is characterized by a negative deviation from Henry's law and shows an accentuated dependence on previous thermal history of the films. In zone II, with a range of activity higher than 0.35, the isotherms follow Flory-Huggin's behavior. In both zones, the diffusional behavior, based on sorption measurements, shows a linear relationship between the diffusion coefficient and the equilibrium sorption concentration of dichloromethane. In the activity range between 0.25 and 0.35, an accentuated discontinuity indicates a possible conformational structural change in the original polymeric matrix of the PEI films induced by the plasticization effect of the dichloromethane. Correlations between transport properties and infrared analysis confirm this proposition. The study of diffusion at different temperatures makes possible the Arrhenius plot analysis of the thermodynamic diffusion coefficient. Two different linear relationships were obtained for zones I and II with activation energiesEa= 30803 J · mol−1andEa= 13613 J · mol−1, respectively. These results indicate that the conformational change (zone I and zone II) observed is typically exothermic and therefore permits the proposition that the structural PEI formed in zone II is more ordered than in the zone I, corroborating the negative deviation of Henry's law. The two Arrhenius plots intersect at 150°C. PEI films annealed at this temperature show a dichloromethane sorption without negative deviation. On the other hand, PEI exhibits aTBtransformation at 70°C related to the conformational changes of the benzimide groups. The results obtained for the PEI films annealed at 150°C confirm that the conformational change observed is related to the benzimide groups. This fact was also confirmed by infrared analysis. Finally, the study of diffusion at different temperatures allowed creation of a temperature-penetrant concentration diagram, which is very useful for visualizing the different behavior zones.