The reactions which take place in the system MgO-MgCl2-H2O at concentrations of MgCl2 solutions up to 45 per cent. and temperatures from 0 to 110 �C have been studied in detail. Four oxychlorides have been found, namely, 9Mg(OH)2.MgCl2.5H2O, 5Mg(OH)2.MgCl2.8H2O, 3Mg(OH)2.MgCl2.8H2O, and 2Mg(OH)2.MgCl2.4H2O ; and an equilibrium diagram has been produced which gives the conditions at which each is stable. It is probable that in the formation of the oxychlorides a complex ion forms first and this reacts with Mg(OH)2, formed by rapid hydration of MgO, to produce amorphous oxychloride gels which slowly crystallize. The reactions in the systems MgO-CaCl2-H2O, CaO-MgCl2-H2O, Ca(OH)2-MgCl2-H2O, CaO-CaCl2-H2O, Ca(OH)2-CaCl2-H2O, and between calcined dolomite and MgCl2, or CaCl2. solutions have been studied briefly. Oxychlorides form in these reactions also. It is therefore suggested that the explanation for the success of the method of Demediuk (1952) for the removal of " unsoundness " of dolomitic limes, and other limes high in magnesium oxide, by the action of salt solutions lies in the conversion of free magnesium oxide to magnesium oxychlorides. The optical properties and stabilities of the magnesium oxychlorides are briefly discussed.