In this paper the thermodynamic properties and the chemical reaction kinetics of the reversible reactions where sorption of water in magnesium hydro carbonates are analysed for thermal energy storage (TES). Depending on the conditions mainly nesquehonite, lansfordite and hydromagnesite may be formed from magnesite, all with a certain heat effect. Magnesite and water vapour can form nesquehonite or lansfordite via reaction (R1) and (R2): MgCO3 + 3H2O(g) ↔ MgCO3∙3H2O ΔH = -1.83 MJ/kg MgCO3, T=298K (R1) MgCO3 + 5H2O(g) ↔MgCO3∙5H2O ΔH = -2.54 MJ/kg MgCO3, T=298K (R2) Compared to other chemical sorption compounds, its advantages are low operating temperatures while they can act as a fire retardant. Experimental data is presented on the reactivity of the dehydration at various temperatures. The rate of dehydration of the nesquehonite is sufficient at low temperatures such as 50 °C and the reaction is about 90 % completed after 120 minutes. Magnesite reaches partial re-hydration to about 37% conversion after 24 hours. For better contact between reagents, mixtures with silica gel were used. A too large amount of water vapour, causing condensation of the water, appears to make the reactions irreversible. The temperatures of operating the process are presented as well as which compounds give an optimal energy storage.