Transport properties of dichloromethane vapor were measured in atactic polystyrene films (aPS), obtained by casting chloroform ando-dichloro-benzene solutions. The diffusional behavior is characterized by three stages, depending on vapor activity. In the first stage, at low activity, the diffusion coefficient is independent of concentration; the second stage, which occurs above a critical activity, is characterized by concentration-dependent diffusion and by the appearance of anomalous sorption curves; in the third stage the strong polymer-solvent interaction induces structural transformation of the samples. The diffusional behavior of the cast films is very similar to that of aPS films obtained by hot pressing, and also the extrapolated parameters are very close, leading to the conclusion that the structural organization is quite similar in both cases. At low penetrant activity the films cast fromo-dichlorobenzene and dried at 70 °C show a lower sorption than the others; the presence of domains in which the chains are differently packed in such a way as to be impermeable to the vapors in the low range of vapor activity is suggested.