Heat-treated PET fibers of different draw ratios (0X, 2.6X) were submitted to different conditions (time and temperature) of chemical treatments in the presence of aniline and aniline plus benzoic acid in order to verify the influence of these chemicals on their structure. Both crystalline structures of these fibers behaved similarly in some aspects to these applied chemical treatments, but differently in others. The major differences would be related to their original structures (control ones) due to the existence or not of previous orientation. The drawn fiber presented a more stable and more complex structure than that of the undrawn one. The crystalline structure of the drawn fibers changed from a two-form and more heterogeneous crystalline structure to a more homogeneous one, which is constituted of smaller and/or less perfect crystallites. The use of benzoic acid in combination with aniline seems to be more effective to plasticize both fibers, especially at higher temperatures and longer times of treatment. At these conditions, the fibers presented swelling and a more effective reduction of their glass transition temperatures (T-g). The swelling and the increase in the chain flexibility might be responsible for the formation of the more homogeneous crystalline structure of the drawn fibers. The swelling promoted not only disorientation of the amorphous regions, but disorientation of their crystalline regions as well. This phenomenon seems to he responsible for the observed decrease in the intensity of the X-ray diffractograms within a given treatment for both fibers. Sonic modulus analysis performed for the drawn fibers confirmed such data.