Display omitted •Dehydration conditions yielding highly reactive MgO for energy storage were found.•Controlling dehydration avoids the need for chemical modification by doping.•Controlled dehydration prevents premature material fatigue.•Impurities in MgO from natural magnesite significantly affect rehydration performance. Systematic variation of the dehydration temperature and time enables the preparation of highly reactive magnesium oxide for thermochemical energy storage purposes. The reactivity of the MgO, resulting from varying dehydration conditions has been studied by a comparative approach, including reactive surface area, particle morphology and reactivity towards rehydration. For the rehydration an in-situ powder X-Ray diffraction setup is used, allowing for continuous monitoring of Mg(OH)2 formation. The outcome of this investigation was subsequently applied to MgO from natural magnesites to assess the impact of impurities in the material on rehydration reactivity.