The thermal transitions of a poly(ethylene terephthalate)/carbon dioxide (PET/CO2) system were investigated by using a differential scanning calorimeter accessorized with a high-pressure DSC cell. It was found that the glass transition temperature of PET decreases with an increase in the CO2 pressure due to the plasticization effect,, which is quite noticeable even at rather low CO2 pressures. The sorbed CO2 enhances the mobility of the chain segments and depresses the crystallization temperature of the PET. The CO2-induced crystallization of PET at high pressure is attributed mainly to the plasticization effect, which causes a lower T-g than room temperature for PET, and hence crystallization of PET can occur at room temperature. The sorbed CO2 was also found to be able to induce the crystallization of PET at temperatures lower than the glass transition temperature of PET. The results of high-pressure DSC were supported by measurements of wide-angle X-ray diffraction (WAXD).