An amorphous poly(ethylene terephthalate) (aPET) and a semicrystalline poly (ethylene terephthalate) obtained through the annealing Of aPET at 110 degrees C for 40 min (aPET-110-40) were treated in carbon dioxide (CO2) at 1500 psi and 35 degrees C for 1 h followed by treatment in a vacuum for various times to make samples containing various amounts of CO2 residues in these two CO2-treated samples. Glass transition and cold crystallization as a function Of the amount of CO2 residues in these two CO2-treated samples were investigated by temperature-modulated differential scanning calorimetry (TMDSC) and dynamic mechanical analysis (DMA). The CO2 residues were found to not only depress the glass-transition temperature (T-g) but also facilitate cold crystallization in both samples. The depressed T-g in both CO2-treated poly(ethylene terephthalate) samples Was roughly inversely proportional to amount Of CO2 residues and was independent of the crystallinity of the poly(ethylene terephthalate) sample. The nonreversing curves of TMDSC data clearly indicated that both samples exhibited a big overshoot peak around the glass transition. This overshoot peak occurred at lower temperatures and was smaller in magnitude for samples containing more CO2 residues. The TMDSC nonreversing curves also indicated that aPET exhibited a clear cold-crystallization exotherm at 120.0 degrees C, but aPET-110-40 exhibited two cold-crystallization exotherms at 109.2 and 127.4 degrees C. The two cold crystallizations in the CO2-treated aPET-110-40 became one after vacuum treatment. The DMA data exhibited Multiple tan delta peaks in both CO2-treated poly-(ethylene terephthalate) samples. These multiple tan delta peaks, attributed to Multiple amorphous Phases, tended to shift to higher temperatures for longer vacuum times. (C) 2009 Wiley Periodicals, Inc. J Appl Polym Sci 113: 3345-3353, 2009