Poly(ethylene terephthalate) (PET) was treated with supercritical fluid (SCF) carbon dioxide to study crystallization of this polymer under SCF conditions. Solid state NMR spectroscopy and wide angle X-ray diffraction; (WAXD) detected dramatic changes in the PET molecular structure and motion after treatment. To fit WAXD data, the powder diffraction pattern for crystalline PET was calculated and is reported here for the first time. The WAXD results indicate an increase in crystallinity from essentially zero in the as-received sample to 62% in the SCF-treated sample. This large increase in the crystallinity of the SCF-treated PET was verified by the NMR relaxation measurements. The X-ray crystallite size, obtained from WAXD, was compared with those obtained from NMR proton spin diffusion measurements. The C-13 signals for aliphatic carbons (centered at 61.5 ppm) in the PET crystalline domain were resolved for the first time in a C-13 cross polarization/magic angle spinning spectrum (CP/MAS) due to the SCF treatment. The C-13 chemical shift tensors for PET were determined experimentally and were also compared to theoretical ab initio calculations. The anisotropic chemical shift data were then interpreted in terms of changes in the molecular conformation in PET as a result of SCF CO2 treatment. It was found that the SCF CO2 treatment is an effective method for enhancing the crystallinity of PET. These results in PET strongly support the previously proposed model for the existence of three motional regimes; crystalline, rigid amorphous, and mobile amorphous.