The changes in the mechanical and thermal properties of cold-crystallized poly(trimethylene terephthalate) during aging at 60 and 80 degreesC were investigated. A significant increase in the tensile modulus and stress at yield and a decrease in strain at yield were observed for both aging temperatures. Differential scanning calorimetry (DSC) scans of the aged sample showed an endothermic annealing peak 10-20 degreesC above the previous aging temperature, the maximum temperature and enthalpic content of these peaks increased with aging time. Dynamic mechanical measurements indicated a relaxation process starting at about 20 degreesC above the aging temperature and correlate with the annealing peak detected by DSC. Density measurements and wide-angle X-ray scattering investigation revealed that neither the crystallinity increased significantly nor did the crystal structure changed. These results were explained by the existence of a third phase besides the crystalline and the 'classical amorphous' which involves oriented and constrained 'non-crystalline' polymer chain sequences close to the crystalline lamellae. (C) 2003 Elsevier Science Ltd. All rights reserved.