Thermally induced solid-state reactions and microstructure changes in a high molar mass, semicrystalline, aromatic diacetylene-containing polyester, poly[2,4-hexadiyn-1,6-ylene terephthalate], were investigated with a combination of laser Raman spectroscopy, differential scanning calorimetry, and wide-angle X-ray diffraction analysis. The study has provided some new insights into the rather complex solid-state reactions in the semicrystalline diacetylene-containing polyester, Results suggest that, in addition to the usual desired solid-state topochemical crosspolymerization in the crystalline region, a certain degree of random crosslinking reaction occurs in the amorphous region, especially when the annealing is carried out above the glass transition. After prolonged annealing or annealing at a higher temperature, a further reaction involving the formed polydiacetylene chains may occur, as evident from the reduction in crystallinity and even complete loss of crystallinity. An attempt has been made to separate the contribution of the topochemical reaction from the. overlapping exothermic activities in the differential scanning calorimetry curves via subtraction, This allows the monitoring of the crystalline-phase solid-state topochemical crosspolymerization.