Local reversibility caused by a reactant (lithium sulfate monohydrate) crystalline skeleton kept after removal of water molecules ("vacancy structure") was studied via dehydration/rehydration cycles repeated under the same conditions. The dehydration reversibility observed indicates the stability of the "vacancy structure". Solid product morphology was studied using electron microscopy. The initial crystal of Li2SO4 . H2O decomposed in the course of dehydration, and thus the reaction zone, consisted of isolated microcrystalline "blocks".