Commercial poly(ethylene terephthalate) yarn was heat set at various temperatures between 100° and 255°C, while held in the taut and slack states, respectively. The lattice parameters and crystal sizes of the as-drawn and heat-set fibers were measured with the help of wide-angle x-ray scattering while the long period was estimated from the small-angle x-ray scattering patterns. Combining the results from these two studies, information was obtained on the type of coupling between the crystalline and amorphous phases. From intensity scans and visual inspection of meridional layer lines of small-angle patterns, the tilt of the lamellar plane normals with respect of the fiber axis was estimated. The formation and reorganization of crystallites on heat setting and their temperature dependence have also been considered. The phase separation was shown to be much more distinct in the samples which were slack annealed at the higher temperatures. The implications of this study for structure-property relationships in polyester fibers are briefly discussed.