The thermal properties of poly(ethylene terephthalate) (PET) conventional fibers and microfibers are measured and compared to bulk samples. It is shown that the glass transition temperature (T-g) of the fibers can be monitored with modulated differential scanning calorimetry (MDSC). The T-g region is about 30degreesC wide and shifted to approximately 110degreesC for conventional as well as for micro-PET fibers. The T-g of these fibers is compared to the T of cold-crystallized bulk samples. Upon crystallization, a lift and even a split up of T-g is observed. The second T-g is much broader and is situated around 90degreesC. This T is related to the appearance of a rigid amorphous phase. In comparison, the mobility of the amorphous phase in fibers is even more restricted. The whole multiple melting profile observed on the fibers is the result of a continuous melting and recrystallization process, in contrast to bulk PET. The heat-set temperature is shown to trigger the start of melting and recrystallization. It is seen in the MDSC as an exotherm in the nonreversing signal and an excess contribution in the heat-capacity signal. (C) 2003 Wiley Periodicals, Inc.