In nitrogen or hydrogen atmospheres the decomposition of hydromagnesite (Mg5(OH)2(CO3)4 · 4H2O) to magnesia occurred in two steps and it stopped at temperatures lower than 750 K. In a carbon dioxide atmosphere, the second decomposition step was divided into two parts and shifted towards higher temperatures. Between the two parts, a strongly exothermic process was observed at 790 K which was identified as the crystallization of magnesite (MgCO3). This phenomenon was also obeserved for nickel-doped hydromagnesite in a carbon dioxide atmosphere. Under hydrogen, the carbon dioxide evolved from nickel-doped hydromagnesite was catalytically reduced to methane in an exothermic process. Due to this reduction the over-all reaction became exothermic.