Sulfide-based all-solid-state lithium batteries are a possible next-generation power source due to their incombustibility and large energy density. A composite comprising LiNi1/3Mn1/3Co1/3O2 (NMC) and 75Li2S·25P2S5 (LPS) glass electrolytes are used as a positive electrode material, which exhibits excellent charge–discharge cycle characteristics. In this study, to understand the origin of exothermal reactions during heating, we investigated structural changes in initially charged NMC–LPS composites mainly by in-situ synchrotron X-ray diffraction measurements and ex-situ transmission electron microscopy observations. We found that significant structural changes occurred in the NMC–LPS composites after heating above 300 °C. NMC decomposed into transition metal sulfides, such as MnS and CoNi2S4, while crystallization occurred in LPS predominantly precipitating a Li3PO4 crystalline phase. The formation of Li3PO4 is attributable to oxidation reactions between Li3PS4 and oxygen, which was generated during NMC decomposition. Thus, the exothermic peaks that were characteristic of the charged NMC–LPS composites are fundamentally related to heat generation that accompanies the formation of transition metal sulfides and Li3PO4 crystalline phases. As a result, when exposed to real cell operating environments, LPS and NMC yield a chemical reaction via heat treatment that leads to the exothermal reactions.