This work is aimed to monitor the physical aging (structural relaxation) of polyester fibers in terms of changes in the number of dye molecules absorbed by aged specimens as a new method and to make some aspects of this complex phenomenon more clear which rarely considered in the literature. Specimens, aged thermally and hygrothermally at the specified temperature and periods, are, thus, conducted to the dye sorption experiments to monitor their physical aging. Both purified and commercial dyes are employed. Independent from dye purities, the number of absorbed dye molecules reduces for both aged thermally and hygrothermally specimens with increasing the aging time because of decreasing of the free volume as well as increasing the crystallinity, revealed by WAXD measurement. However, the rate of aging of hygrothermally aged specimen is faster than that of the thermal one due to the role of water as a plasticizer in the polymeric bulk. The non-exponential character of the relaxation is exhibited by fitting of KWW (Kohlrausch-Williams-Watts) stretched exponential function and two additive exponential models into the relaxation data, constructed from the measuring of the changes in the number of absorbed dye molecules during aging. Using FTIR spectroscopy a correlation between the number of absorbed dye molecules and the amount of trans conformers for the sample upon thermal aging is found which shows the method yields reliable information on physical aging of the polymer. (C) 2016 Elsevier Ltd. All rights reserved.