The dehydration and subsequent rehydration of calcium oxalate monohydrate has yet to find application in thermochemical energy storage. Unlike for many other salt hydrates, complete reversibility of the dehydration-rehydration reaction was observed. Additionally, it was found that the rehydration temperature is strongly affected by the water vapour concentration: Full reversibility is not only achieved at room-temperature, but, depending on the water vapour concentration, at up to 200 degrees C. This allows isothermal switching of the material between charging and discharging by a change of the H2O-partial pressure. Cycle stability of the material was tested by a long-term stress experiment involving 100 charging and discharging cycles. No signs of material fatigue or reactivity loss Were found. In-situ powder X-ray diffraction showed complete rehydration of the material within 300 s. The experimental findings indicate that the CaC2O4 center dot H2O/CaC2O4 system is perfectly suited for technical application as a thermochemical energy storage medium. (C) 2016 Elsevier Ltd. All rights reserved.