The amorphous structure changes in poly(ethylene terephthalate) (PET) induced by annealing under dry and wet conditions were discussed in terms of amorphous density (rho(a)) and dye uptake properties. The rho(a) value of PET decreased with increasing annealing temperature, but it conversely increased at annealing temperatures above 210 degrees C for the dry treatment and above 170 degrees C for the wet treatment. With decreasing rho(a) from 1.35 to 1.31 g cm(-3), the conformation of amorphous molecules changed from the gauche to trans form. The rho(a) value conversely increased on annealing at higher temperatures; however, the dye exhaustion continuously increased. The ratio of the absorbance of the alpha band to that of the beta band of the basic dye crystal violet (alpha/beta ratio), indicating the degree of dye aggregation, in PET decreased with a decrease in rho(a). The degree of dye aggregation further increased when rho(a) conversely increased on high temperature annealing. These results suggested that the free volume content of PET increased as a result of conformational change from the gauche to trans form in amorphous molecules with increasing annealing temperature, and finally the increased free volume converted to microvoids around 1.31 g cm(-3) of rho(a). The dye exhaustion increased with increasing annealing temperature according to microvoid formation.