The interrelation between dyeing and the thermal properties of micro and conventional poly(ethylene terephthalate) fibers is studied with conventional and modulated differential scanning calorimetry. X-ray diffraction, density, and birefringence studies are used to confirm the obtained results. It is shown that three studied anthraquinone dyes, in contrast to three studied benzodifuranone dyes, act as plasticizers for the fibers. A comparison between fibers and partially crystallized bulk samples is made. The specific fiber morphology makes the fibers more susceptible to plasticization than are bulk samples. This ability of a dye to lower the glass transition of the fibers will influence the dye diffusion and this information is needed to optimize the dyeing process for a specific dye-fiber combination. Further, the presence of an anthraquinone dye in the fiber alters the melting endotherm by changing the stability of the original crystals. The onset of the melting and recrystallization process is lowered. Although still well above the dyeing temperature, this lowering may be critical for the dimensional stability of the fabrics during any subsequent high-temperature process. A first investigation of the effect of variations in the thermal properties on the obtained color strength after dyeing is reported. (C) 2003 Wiley Periodicals, Inc.