Enthalpies of hydration of low-temperature hydrotalcite(HT) metaphases have been measured by means of the adiabatic water-vapor absorption calorimeter. The metaphases were characterized by the Mg/Al ratio of 3:1 and CO3 2- interlayer group. The hydration heat for HT-B (partially dehydroxylated HT with bidentate bonded CO3 2-) is 2-3 times larger than that of HT-D (dehydrated HT), if they are treated at the same dehydration temperature. Large absolute values of hydration heat, namely 398±4 and 508±10Jg-1, were obtained for the HT-B at respective low dehydration temperatures, 70 and 110°C. This may promise the HT-B to be a new prospective heat exchanger usable at relatively low temperatures.