The enthalpies of solution of eight hydrated magnesium berates in approximately 1 mol.dm(-3) aqueous hydrochloric acid were determined. From a combination of these results with measured enthalpies of solution of H3BO3 in HCl(aq) and of MgO in aqueous (hydrochloric acid + boric acid), together with the standard molar enthalpies of formation of MgO(s), H3BO3(s), and H2O(1), the standard molar enthalpies of formation of eight hydrated magnesium berates were obtained.