Poly(ethylene terephthalate) (PET) fibers of different draw ratios (as spun and drawn 2.8X) were submitted to different heat settings. Subsequently, they were treated in benzoic acid solutions at different concentrations and for different times of exposition. The plasticizing effect on the morphology of these heat-setted fibers due to the different treatments in benzoic acid solutions was studied by differential scanning calorimetry (DSC) and dynamic and mechanical thermal analysis (DMTA). The DSC experiments revealed the appearance of a premelting peak (pml) around 130 degrees C, when the applied heat settings were carried out for 8 h in the presence of boiling water, and around 150-160 degrees C for the dry heat setting for 7 h at 130 degrees C followed by 1 h in the presence of boiling water. These premelting peaks have been associated with the melting of smaller and imperfect crystals present in the amorphous region. Also, the analysis of the main melting peaks revealed that the undrawn fiber presented a wider distribution of smaller crystals than the drawn one. Their subsequent treatments in the benzoic acid solutions revealed through the DSC thermograms that this first premelting peak tends to disappear as the concentration and time of benzoic acid treatment increase. Also, a new premelting peak (pm2) appears in the DSC thermograms at temperature around 180 degrees C. It seems that a dislocation of the pml peak is occurring toward higher temperatures and is being transformed in this new premelting peak. The DMTA revealed the appearance of an alpha(c) transition in the temperature range from 140 to 160 degrees C and this alpha(c) transition seems to be related to the relaxations of the new crystallites formed due to the plasticization effect of the benzoic acid. Also, the second premelting peak seems to be related to the fusion of such crystals. Therefore, a complex change in the morphology of the studied PET fibers due to the benzoic acid action has been revealed.