The effect of phase structure on the sorption and de-sorption of methylene chloride in poly(trimethylene terephthalate) was studied using low and high-crystallinity pellets (22 and 54%, respectively) as the starting material. Pellets were subjected to sorption for 12 days followed by de-sorption up to 7 months. During this cycle samples were removed periodically, weighed on a microbalance to assess sorption kinetics and analyzed by thermal analysis, thermogravimetry and differential scanning calorimetry, to characterize phase structure. Sorption into low-crystallinity pellets appeared linear with respect to the square root of time and exhibited an overshoot above the final equilibrium value. Sorption into the high-crystallinity pellets suggested an induction time on the order of 1-2 h. Thermal analysis showed complex changes taking place, especially in low-crystallinity samples. Up to four phases were identified with increasing stability: amorphous, rigid amorphous, solvent-induced crystal and crystal formed from the melt.