The effects of draw conditions were studied for initially amorphous melt-spun poly(ethylene terephthalate) fibers in the presence of subcritical and supercritical (SC) CO2. Both in situ and posttreatment mechanical behavior along with morphological characteristics were investigated. Fibers soaked in subcritical CO2 could be drawn to 30% higher draw ratios (DRs) compared with fibers that were cold-drawn. In situ force response measured with a custom apparatus showed that fibers in subcritical CO2 had no measurable resistance to deformation until strain hardening occurred. In contrast, fibers drawn in SC CO2 displayed a yield response, a significant decrease in ductility, and a significant difference in postyield behavior. Fibers drawn in subcritical CO2 showed slightly lower tensile properties compared with cold-drawn samples whereas fibers treated in SC CO2 had much lower tensile properties because of the limited DR achieved. X-ray diffraction studies indicated that CO2 enhances the development of the crystalline phase compared with cold-drawn samples.