A new method to simultaneously monitor the development of crystalline, mobile amorphous and rigid amorphous fractions during cooling from the melt in semicrystalline polymers is here presented. The method, which allows the determination of the temperature evolution of the three nanophases, is the development of a previous enthalpy-based procedure founded on a two-phase description of the semicrystalline polymers, with only one crystalline phase and one amorphous phase accounted for. The proposed method has been applied to determine the development of the nanophase structure in poly(ethylene terephthalate) (PET) upon cooling from the melt. In addition the temperature-dependent enthalpy of the rigid amorphous phase has been calculated. For the cooling rates investigated, it was found that the rigid amorphous fraction starts to vitrify when the crystallization process is almost finished and continues further on cooling to the glass transition of the bulk amorphous phase. A possible connection between secondary crystallization and initial vitrification of the rigid amorphous phase has been hypothesized for PET. (c) 2007 Elsevier B.V. All rights reserved.