Recent processing advances, developed to improve barrier properties and thermal stability of oriented poly(ethylene terephthalate) (PET), present a need to further study and understand the resultant material changes. The mechanical, optical, and infrared characteristics of PET sheets, previously oriented and subsequently thermally crystallized (under unconstrained conditions) have been investigated. The effects on properties, with respect to the initial mode of extension and extension ratio, have been studied. Results indicate that mechanical properties of this form of PET are dependent on the initial extension ratio, in addition to the initial mode of extension. Among thermally crystallized samples, optical property variations were found to be strong functions of initial extension ratios as well as the levels of thermal crystallinity achieved. Unoriented samples became completely opaque at low levels of thermally induced crystallinity, while highly oriented samples remained optically clear even with high levels of additional crystallinity induced by thermal exposure. Moderate levels of initial orientation were also found to inhibit the development of haze, resulting from additional thermal crystallization, although not to the extent observed for highly oriented samples. Crystal growth resulting from unconstrained thermal exposure was monitored using internal reflectance infrared spectroscopy. This growth was found to selectively occur in the directions of polymer chain orientation (alignment) for samples prepared under conditions of both uniaxial and equibiaxial stretching.