The solubility equilibria of the ternary system Mg(OH)(2)-MgCl2-H2O were determined at temperatures from 298 to 393 K applying equilibration periods of up to 3.5 years. As a result, four thermodynamically stable magnesium chloride hydroxide hydrates (Sorel phases) exist in the ternary system within the investigated temperature range. These are the 3-1-8 phase [3Mg(OH)(2)center dot MgCl2 center dot 8H(2)O], the 9-1-4 phase [9Mg(OH)(2)center dot MgCl2 center dot 4H(2)O], the 2-1-4 phase [2Mg(OH)(2)center dot MgCl2 center dot 4H(2)O], and the 2-1-2 phase [2Mg(OH)(2)center dot MgCl2 center dot 2H(2)O]. The also known 5-1-8 phase [5Mg(OH)(2)center dot MgCl2 center dot 8H(2)O] was found to be metastable in the solid-liquid system. With this work, a reliable solubility data set is now available, for example, to prove the long-term stability of magnesia building materials in the presence of salt-bearing media, a challenging demand on the material in a special application as a barrier construction material in salt formation.