CaMnO3 oxide can be considered a promising candidate for high temperature thermochemical heat storage, since it is able to release oxygen in a wide temperature range (800-1000 °C) at different oxygen partial pressures (pO2) suitable for Concentrated Solar Power (CSP) plants. Moreover, it is composed of earth abundant, inexpensive, non-toxic elements. However, it undergoes decomposition at pO2<0.01 atm and at temperature above 1100 °C. In order to overcome this limitation and to extent the operating temperature range, in this study B-site doping with Fe was used as approach for preventing decomposition. The reaction enthalpy was measured through equilibrium non-stoichiometry curves so that the heat storage capacity could be evaluated. It was demonstrated that Fe-doping prevented CaMnO3 decomposition up to 1200 °C at pO2=0.008 thus widening the operating temperature range and the oxygen reduction extent. In addition, the heat storage capacity (ΔH (kJ/molABO3)) of Fe-CaMnO3 (∼324 kJ/kgABO3) is remarkably higher than that of the un-doped CaMnO3 (∼250 kJ/kgABO3).