In this study, we investigated the mechanistic features and kinetics of the thermal decomposition of solids accompanied by liquefaction as exemplified by the thermal dehydration reactions of monosodium L-glutamate monohydrate (MSG-MH). The thermal dehydration of MSG-MH occurs via two mass-loss processes comprising the elimination of crystalline water and intramolecular dehydration. Multistep kinetic behaviors and the liquefaction during both thermal dehydration processes were evidenced by systematic thermoanalytical measurements and in situ microscopic observations. During the thermal dehydration of crystalline water, the liquefaction of the surface product layer occurred midway through the reaction, and the subsequent reaction proceeded with a geometrical constraint, where the solid reactant was covered by a liquid surface layer, affording a solid anhydride. The intramolecular dehydration of the solid anhydride yielded a liquid product on the surface of the reacting particles, and the internal solid reactant dissolved in the liquid product. Subsequently, the intramolecular dehydration proceeded in the liquid phase to afford liquid sodium pyroglutamate. The net kinetic behavior of the physico-geometrical reaction steps in each thermal dehydration process was revealed using kinetic approaches based on cumulative and conjunct kinetic equations. The advanced kinetic approaches employed to reveal the specific kinetic features of the heterogeneous reaction processes in solid-liquid-gas systems are described in this article.