Abstract <p>Changes in the structure and mechanical properties of poly(ε-caprolactone) films and fibers during their exposure to iodine vapors have been studied. The limit content of iodine in polymer samples has been reached in 15–18 h, being about 5% by weight. The value of the diffusion coefficient of I₂ in semicrystalline samples of poly(ε-caprolactone) has been determined to be 1.7 × 10⁻¹¹ cm²/s. It has been found that the process of iodine sorption into the polymer is accompanied by a gradual decrease in the value of the long period from 16 to 12 nm. Simultaneously, degree of crystallinity and strength of poly(ε-caprolactone) samples have been changed nonlinearly. It has been suggested that the changes in structural and mechanical parameters at the initial stages of iodine sorption are associated with the formation of a complex with charge transfer between iodine and the ester group of the polymer. At later stages, as a result of destruction of poly(ε-caprolactone) and decrease in the number-average molecular weight to 9 × 10³, crystallization of the polymer has occurred in the amorphous layers, leading to an increase in the degree of crystallinity to 75–79% and sharp embrittlement of the samples.</p>