•The morphology of Mg(OH)2used for thermochemical energy storage is shown to be dependent on the salt precursor used in the precipitation of the material.•A large difference in onset temperature of these precipitated materials is reported, with comparable thermal capacity.•Mg(OH)2precipitated from magnesium acetate, Mg(CH3COO)2, and ammonium hydroxide, NH4OH, is shown to dehydrate at 47 °C lower than high purity commercial Mg(OH)2with a comparable energy storage in the reaction.•Mg(OH)2precipitated from magnesium chloride, MgCl2, is also shown to be a possible thermochemical energy storage material with reduction in onset temperature of 26 °C, whilst magnesium sulphate, Mg(SO)4, as a precursor leads to a higher onset temperature and a lower heat of reaction. Magnesium nitrate, Mg(NO3)2, as a precursor has a reduced onset temperature but dehydrates far slower than either Mg(CH3COO)2or MgCl2derived material.