Isothermal solvent diffusion phenomena for dioxane in polyethylene terephthalate (PET) film were analyzed. The change in diffusion behaviors was investigated in terms of the microstructural change of PET treated by drawing and by solvent-induced crystallization(SINC) using methylene chloride vapor. Using the Kelly-Bueche equation and adopting the classification scheme of Vrentas and the experimental results of Thomas and Durning, the diffusion behaviors of dioxane in PET can be classified as typical Fickian behavior above T , Case II above T among non-Fickian behaviors, and typical non-Fickian behavior above T , Case II above T among non-Fickian behaviors, and typical non-Fickian behavior above t According to changing the microstructure of PET. Also, the diffusion of dioxane in amorphous PET results in cry stallization due to good compatibility between PET and dioxane over whole investigated temperature range.