Metal oxides with multiple accessible oxidation states are considered as promising candidates for high temperature thermochemical energy storage materials. To shed light on the chemical processes involved in redox thermochemical energy storage materials, in-situ powder diffraction was used in combination with atmospheric control to investigate the redox-reactions of Co3O4, MnO2 and PbO2 under various conditions. Thermogravimetry and differential scanning calorimetry under the same conditions provided information on heat-flows and mass-changes. In contrast to theoretical thermodynamic considerations, only Co3O4 and Mn2O3/Mn3O4 (originating from MnO2) were found fully reversible. In the case of PbO2 for none of the numerous intermediate phases any kind of reversibility was observed. The effect of the O-2-concentration in the reactive atmosphere was most distinct for the Mn2O3 system, notably affecting the reduction/oxidation temperatures, whereas for the Co3O4 system only a moderate influence of the O-2 concentration was found. Based on the stability of the intermediate phases under various atmospheres, an isothermal TCES-cycle for Co3O4 and Mn2O3 was investigated, triggering the redox-process by an abrupt change of reactive-gas atmosphere. The fast reaction rate combined with a significant down-shift of the reaction temperatures compared to an isokinetic redox reaction suggests application as a chemical heat pump, as well towards a broadened operational profile not only in combination with concentrating solar power plants, but also with e.g. recycling of industrial flue gas heat. (C) 2017 Elsevier Ltd. All rights reserved.