The dye (Crystal Violet) exhaustion of poly(ethylene terephthalate) (PET) increased with increasing annealing temperature for both the dry and wet conditions. The ratio of the absorbance of the α-band (600nm) to that of the β-band (560nm) (α/β ratio) has been used as an indication of the degree of aggregation of the dye. The value of α/β ratio gradually decreased with an increase in dye exhaustion of PET. This suggests that the dye molecules partially exist in an aggregated form at high dye concentration in PET. On the other hand, the density and the degree of crystallinity of PET also increased with increasing annealing temperature. Based on the two-phase model, the amorphous density of PET was calculated from the observed density and degree of crystallinity. The amorphous density decreased as crystallization increased under both the dry and wet conditions. Under the wet annealing condition, the amorphous density decreased faster than that of PET during dry annealing. The structure of the amorphous region of PET became sparse as the crystallization proceeded, especially the amorphous structure of the wet-annealed PET significantly changed as a result of the plasticizing effect of adsorbed water. The increase in dye exhaustion of PET with increasing annealing temperature was explained well in terms of the amorphous structure.