Reflection-absorption infrared spectroscopy was used to study the crystallization behavior of poly(ethylene terephthalate) (PET) ultrathin films. The crystallinity of ultrathin films was estimated by the fraction of trans conformers of PET. The isothermal and nonisothermal crystallization kinetics of ultrathin films with different thicknesses were investigated. The thinner PET film showed slower kinetics during isothermal crystallization than the thicker film. Moreover, the final crystallinity of films with various thicknesses were reduced with decreasing thickness. An Avrami equation was used to fit the acquired results. The Avrami exponents decreased with the film thickness. As for the nonisothermal crystallization, the cold-crystallization starting temperature shifted to a lower temperature as the film thickness increased. The influence of the substrate on the crystallization kinetics of the films was also studied. The half-crystallization times and final crystallinities of ultrathin films adsorbed onto a self-assembled-monolayer-treated surface and an untreated substrate were clearly different, although their thickness dependence was similar. (C) 2004 Wiley Periodicals, Inc.