TiO2/SiO2 Sol was intercalated into montmorillonite (MMT), which was pretreated with polyvinylpyrrolidone (PVP). A series of poly(ethylene terephthalate) (PET)/MMT hybrids were prepared using the obtained MMT as polycondensation catalysts. X-ray diffraction (XRD) results proved that MMT dispersion states could be controlled by the amount of TiO2/SiO2 Sol that was incorporated into MMT, ranging from agglomeration to exfoliation. The crystallization behavior of PET/MMT composites synthesized in this study was characterized by differential scanning calorimeter (DSC), polarized optical microscope (POM) and scanning electron microscopy (SEM) to clarify the effects of clay treatment and its dispersion state on the crystallization behavior of the PET substrate. The results indicated that MMT treated with less PVP would retain relatively higher nucleation efficiency while when MMT containing more PVP, the nucleation effect of MMT became weaker. If MMT formed big grains within PET substrate, the disturbance of the growing crystals was negligible; but the exfoliation of MMT layers would greatly magnify the spatial constraint which would slow down the crystallization process of PET matrix. For certain exfoliated PET/ MMT composites, the crystallization rate was even lower than that of pure PET though clay content was low. (C) 2009 Wiley Periodicals, Inc. J Appl Polym Sci 114: 2327-2338, 2009