A series of fibers of poly(ethylene terephthalate) with different physical structures, varying from amorphous to 36% crystalline, has been investigated by C-13 solid-state NMR measurements. Several relaxation times, i.e., relaxation times in the rotating frame (T1rho(H-1), T1rho(C-13)) and H-1-C-13 cross-polarization transfer times (T(CH)), have been studied. C-13 CP/MAS spectra show that, especially with respect to the ethylene and carbonyl carbons, the chemical shift for carbons in ordered structural surroundings ("NMR crystalline") is higher than that for carbons in unordered surroundings ("NMR amorphous"). Also the width of the distribution of chemical shifts in an ordered structure is smaller than that in a less ordered structure (narrow vs broad resonance line). Analysis of the line-shape changes during the T1rho(C-13) relaxation process of semicrystalline yarns showed that the NMR amorphous phase relaxes with two time constants. A three-region model composed of NMR crystalline, rigid NMR amorphous, and mobile NMR amorphous regions is proposed. The sizes of the rigid domains in semicrystalline PET yarns estimated from T1rho(H-1) measurements correspond well with the crystal sizes as determined with X-ray diffraction. In a yarn, which is almost completely amorphous according to X-ray measurements, fast and slow ethylene and aromatic group motions, relative to a correlation time of 5 x 10(-6) s, occur. From T1rho(H-1) experiments it can be concluded that both mobile and rigid regions in the amorphous yarn have dimensions smaller than 50 angstrom. T(CH) measurements reveal that ethylene and aromatic groups in the mobile amorphous domains of all yarns undergo large-amplitude motions. The principal elements of the chemical shift tensor of aromatic carbons remain independent of temperature up to at least 327 K, indicating that, in crystalline regions in semicrystalline yarns and in rigid parts of the amorphous yarn, phenyl reorientations have a very small amplitude (not exceeding approximately 5-degrees).