Structural change in the crystallization process of polyoxymethylene (POM) cooled from the molten state has been investigated by the measurements of infrared spectra and small-angle (SAXS) and wide-angle X-ray scatterings (WAXS). When the melt was cooled slowly, the infrared bands characteristic of a folded chain crystal (FCC) were observed to appear around 156 degreesC. Below 140 degreesC, the infrared bands intrinsic of an extended chain crystal (ECC) were detected to increase in intensity. In the SAXS measurement, the peak (L-1) corresponding to a stacked lamellar structure with the long period of ca. 14 nm was found to grow in parallel to the growth of infrared FCC bands. In the temperature region of the observation of infrared ECC bands, the new peak (L-2) of long period of ca. 7 nm was found to appear and the intensity exchange occurred between the L-1 and L-2 peaks, that is, with decreasing temperature the L-2 peak increased the intensity and its height became comparable to the L-1 peak height. By combining all these experimental data, a model to illustrate the formation process of lamellar stacking structure has been presented. After the appearance of stacked lamellar structure of 14 nm long period from the melt, new lamellae are created in between the already existing lamellae and the long period changes to the half value, 7 nm. Some of molecular chain stems in a lamella are speculated to pass through the adjacent lamellae to form a bundle of fully extended taut tie chains, which are considered to be observed as the infrared bands characteristic of ECC morphology. Although the POM samples used in this experiment may contain small amount of low-molecular-weight macrocyclic component, it was not plausible judging from the various experimental data to assign the secondarily observed 7 nm SAXS peak to the repeating period originating from the stacked structure of macrocyclic compounds. (C) 2003 Elsevier Science Ltd. All rights reserved.