The isothermal cold crystallization of poly(ethylene terephthalate) was investigated by simultaneous small and wide angle X-ray scattering (SAXS and WAXS) and dielectric spectroscopy (DS). By this experimental approach (SWD), simultaneously collected information was obtained about the specific changes occurring in both crystalline and amorphous phases during crystallization. The main features which are directly derived from our experiments can be explained assuming the formation of a heterogeneous multiple lamellar population arrangement. The rigid amorphous phase can be associated with the intra-lamellar stack amorphous phase. The restriction of the amorphous phase mobility mainly Occurs in the inter-lamellar stacks regions probably due to the formation of secondary lamellae. (C) 2004 Elsevier Ltd. All rights reserved.