The crystallization kinetics of previously oriented poly(ethylene terephthalate) (PET) sheets have been studied under unconstrained conditions, as functions of stretch ratio, mode of extension, and crystallization temperature. Isothermal and dynamic crystallization conditions have been utilized for the determination of half-times of crystallization and activation energies, respectively. Results indicate that rates of crystallization of pre-oriented PET increase substantially, with increasing stretch ratios. Half-times of crystallization, at a given isothermal temperature, were found to decrease from a few minutes to fractions of seconds. Similarly, the activation energies of thermal crystallization have been shown to decrease from 75 kcal/mol for unoriented PET, to less than 20 kcal/mol for highly oriented material. These changes are most apparent among samples with low to moderate levels of orientation. Highly oriented samples have proven to be less sensitive, yielding similar extents of thermal crystallization, regardless of specific initial extension ratios. These samples have also shown less sensitivity to temperature of crystallization, exhibiting only slightly higher levels of crystallinity as a result of exposure to increasing temperatures in the range from 100 degrees C to 120 degrees C.