Storing and temperature upgrading of heat energy at 773 K by means of a CaO-CO2 reaction seems very attractive. One way of storing the reaction product CO2 gas is in the form of an other carbonate by letting it react with a metal oxide, such as MgO or ZnO. Two metal oxides, MgO and (CaMg)O-2 were selected for CO2 storage, with the CaO-CO2 energy storage system as CaO-CO2-MgO and CaO-CO2-CaMgO2 systems, and studies on CaO-CO2, MgO-CO2 and CaMgO2-CO2 reactions over the temperature range 773-1073 K were carried out. As the CaO-CO2 system is used not only for storing the heat energy but is also expected to perform as a heat transformer, verification of the temperature upgrading ability is made by means of a lab-scale adiabatic reactor, Pseudo-steady state temperature and pressure existed during the experiments, which correspond quite well with Haul’s equilibrium dissociation values of CaCO3, illustrating that upgraded temperatures of above 1273 K can be achieved by adjusting the carbonating CO2 gas pressure to around 500 kPa.