Isotope effect on the structural evolution process in the melt-isothermal crystallization phenomenon of polyoxymethylene (POM) has been studied on the basis of the time-resolved simultaneous measurements of FTIR spectra, small-angle X-ray scattering (SAXS) and wide-angle X-ray diffraction (WAXD) for a series of copolymers with the different D/H contents. The quantitative analysis of FTIR band intensities for a series of POM D/H random copolymers revealed the critical sequence lengths or the minimal helical segmental lengths necessary for the detection of the IR band intensity. The quantitative analysis of the time-resolved SAXS and WAXD data revealed the formation of the domains of higher density in the melt, the increasing correlation between these domains and the growth of the crystalline lamellae. The time-resolved FTIR data gave the generation and growth of the helical segments in parallel. By combining all the data collected simultaneously, the following observations have been deduced about the structural evolution process from the melt: (1) immediately after the temperature jump, some portions of random coils in the melt regularize into the domains consisting of the aggregation of the short helical segments of 3-5 turns, (2) these domains approach each other to have stronger correlation, (3) and they form the crystalline lamellae. These lamellae grow furthermore by adding the helical segments on the surface. (4) When the degree of supercooling (the difference between the equilibrium melting point and the crystallization temperature) is high, the insertion of new lamellae into the originally-generated lamellae occurs. These structural evolution processes were found to be common to all the isotopic species including a normal POM (POM-H), a fully-deuterated POM (POM-D) and the random copolymer of deuterated (CD2O) and hydrogeneous (CH2O) monomeric units. However, these processes were found to occur at the different rate depending sensitively on the D/H content: the slowest POM-D << D/H-random copolymer << the fastest POM-H. These different structural evolutions were found to be scaled systematically by shifting along the time axis over a wide range of the degree of supercooling. (C) 2016 Elsevier Ltd. All rights reserved.