High temperature powder X-ray diffraction (HTXRD) patterns of Mg/Al-layered double hydroxides (LDH) with interlayer carbonate (Mg/Al/CO3-LDH) indicate that a solid phase having a hydrotalcite-like layered structure (Phase I) is not stable when the temperature of solid samples is elevated. Another phase (Phase LI) grows eminently and simultaneously with the degradation 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. Both MgO and a stable spinel phase (MgAl2O4) finally appear in the temperature range of 400-1000 degrees C. Two endotherms associated with weight loss are observed in differential thermal analysis/thermal gravimetry (DTA/TG). Phase II appears at the endotherm of lower temperature and disappears at that of higher temperature. The temperature at which Phase II appears depends on atomic ratio Mg/Al in Mg/Al/CO3-LDH whereas the temperature at which Phase II disappears does not. Phase II is metastable at room temperature and it transforms to an amorphous phase slowly after days of storage in dry conditions although it re:turns quickly to Phase I in a wet atmosphere. Solid state chemistry of the synthetic Mg/Al/CO3-LDH is discussed by means of HTXRD, DTA/TG and X-ray photoelectron spectroscopy (XPS).