Differential scanning calorimetry (DSC) analysis has been performed on modified and commercial amorphous samples of poly(ethylene terephthalate) (PET) crystallized by high pressure carbon dioxide (CO2). Two endothermic peaks are present in the DSC scans of all the carbon dioxide-treated samples. A qualitatively analogous behavior has been detected in the case of amorphous samples heat treated at temperatures slightly exceeding the glass transition temperature of virgin material. Wide angle X-ray scattering analysis has confirmed the structural analogies between samples CO2 crystallized at 50-degrees-C and thermally crystallized slightly above T(g). A differential scanning calorimeter capable of working at high pressure of CO2 has been adopted in order to examine the effect of carbon dioxide on the crystallization temperature range.