The structural formation behaviors during molding processes for practical polymers, such as crosslinking for rubbers and injection molding for thermoplastics, are often not well understood. In this Part ‡W , in-situ observation of the crosslinking and crystallization processes using synchrotron X-ray sources was introduced as a useful method to elucidate the temporal changes in structures, by adjusting the time resolution to match the processing times. Crosslinking reaction of sulfur-based rubber compounds was investigated by time-resolved X-ray adsorption fine structure (XAFS) measurements. The Zn K-edge XANES (X-ray absorption near-edge structure) spectra obtained during the crosslinking revealed that the decreasing behavior of the amount of zinc oxide (ZnO) give a good indicator of the crosslinking reaction, involving the transformation of ZnO into zinc sulfide (ZnS). In-situ observation of crystallization processes of isotactic polypropylene (iPP) during the injection molding was investigated by time-resolved wide-angle X-ray scattering (WAXS) measurements. The changes in the WAXS patterns under various molding conditions such as mold temperature and injection speed were directly related to the crystallization behaviors of iPP.