The heats of hydration of hydrotalcite (HT) metaphases differing in composition were measured using an adiabatic hydration calorimeter. The metaphase considered in this paper is referred to as HT-B (partially dehydroxylated HT with bidentate-bonded CO 2Γ
3 ). Several factors were found to be decisive for the properties of HT-B as a heat adsorbent, namely (i) the ratio Mg/Al ΒΌ 2:1, 3:1 and 3.7:1; (ii) CO 2Γ 3 or SO 2Γ 4 being the interlayer anions; and (iii) the temperature of aging of the initial HT samples (90 or 20 Β°C). The temperature of transformation of HT into HT-B (from 200 to 250 Β°C) was found to vary with changing the above-mentioned factors. The dehydration temperature before the calorimetric measurement was in the interval 50-140 Β°C. The metaphase HT-B is a very suitable heat adsorbent due to its large surface area and the low temperature of dehydration. The values of the heat of hydration are highest for the samples aged at 90 Β°C and having CO 2Γ 3 as interlayer anion. Their sorption ability is almost independent of the Mg/Al ratio at higher dehydration temperatures, while at 70 Β°C the sample with the highest value Mg/Al ΒΌ 3.7:1 (with an absolute heat of hydration of 450 J g Γ1 ) becomes much more suitable as heat adsorbent than the forms with lower Mg contents.