A solid phase having hydrotalcite-like layered structure (Phase I) of Mg and Al layered double hydroxide with interlayer carbonate (Mg/Al/CO3-LDH) is not stable in in situ high temperature powder X-ray diffraction patterns when the temperature of solid samples is elevated. Another phase (Phase TI) grows with the decomposition of Phase I. Phase II has smaller basal spacing than that of Phase I and is not stable when the temperature is elevated above 380 degrees C. The temperatures at which Phase II appears and disappears agree with temperatures of two endotherms that are observed in differential thermal analysis/thermalgravimetry. The temperature at which Phase II appears decreases with the increase of atomic ratio Mg/Al in layers of Mg/Al/CO3-LDH, whereas the temperature at which Phase LT disappears does not.