Isothermal crystallization kinetics of poly(ethylene terephthalate) (PET) in its blends with poly(ether imide) (PEI) has been investigated by differential scanning calorimetry (d.s.c.). A modified Avrami analysis considering both primary and secondary crystallization was employed to extract the kinetic behaviour of these two crystallization stages. The crystallization rate constants at various crystallization temperatures (T-c) were obtained from the analysis for different PET/PEI blend compositions. A Hoffman-Lauritzen analysis was conducted for both primary and secondary crystallization. An average value of the nucleation constant K-g approximate to 2.89 x 10(5) was obtained from the analysis, and the fold surface free energy sigma(c) was then calculated to be 70.6 erg cm(-2).