The crystallization behaviour of poly(ethylene terephthalate) in poly(ether imide) (PEI)/poly(ethylene terephthalate) (PET) blends was investigated using Fourier transform infrared spectroscopy. Thin films of PEI/PET blends were prepared by solution blending, and many characteristic bands for the PET crystalline region were observed as a function of the annealing time of PEI/PET blends at 225 degrees C. It was found that the C=O stretching band shifted up to 5 cm(-1) in PEI/PET blends due to the increase in crystallinity of PET. This C=O band shift is in agreement with the increase for the characteristic crystallization band of PET, and the band shift width is proportional to the PET weight fraction in PEI/PET blends. Crystallization of PET occurs immediately by annealing for PEI/PET blends with a high PET weight fraction (2/8 and 4/6 PEI/PET blend composition). In the case of PEI/PET blends with a low PET weight fraction (6/4 and 8/2 PEI/PET blend composition), crystallization of PET occurs in the PET-rich phase only after the amorphous phase is separated into PET-rich and PEI-rich phases.