Supercritical carbon dioxide (scCO(2)), an environmentally friendly solvent, can change the fine structure of fibers depending on treatment temperature and pressure. This article presents the results of an investigation into the effects of drawing temperature, scCO(2) exposure and tension on structural changes, and mesomorphic transitions in partially oriented yarn (POY) and oriented poly(ethylene terephthalate) (PET) yarns. Samples of POY and PET yarns were uniaxially drawn at different temperatures to obtain filaments with different structures for subsequent exposure to scCO(2) in the presence and absence of tension at a temperature of 80(o)C and under a pressure of 220 bar. Structural investigation of the samples was performed by combining the results obtained from differential scanning calorimetry, Fourier transform infrared, mechanical deformation, and density and birefringence measurements, all of which exhibited a good correlation. The fractional free volume in the PET samples was considered for interpreting structure-property relations. Results showed that the development of a transient structure strongly depends on process temperature and that for an identical draw ratio, PET yarns drawn at 23(o)C are significantly more oriented than those drawn at 68(o)C. Two crystallization mechanisms, that is strain-induced and thermal-induced crystallization, are involved that are frequently used to explain the structural changes during yarn deformation at different temperatures. The degree of crystallinity and orientation factors for PET yarns does not proportionally increase with increasing temperature of the drawing process; indeed, the highest value is obtained at a temperature of 100(o)C, whereas the lowest is observed at a temperature of 68(o)C, which is near the glass transition temperature. Tension under scCO(2) exposure also produces significant effects in term of causing structural changes. POLYM. ENG. SCI., 55:2030-2041, 2015. (c) 2014 Society of Plastics Engineers