X-ray, differential thermal analysis, and dehydration methods have been used to study the magnesium oxychlorides 3Mg(OH)2.MgCl2.8H2O and 5Mg(OH)2.MgCl2.8H2O formed at temperatures below 100 �C, and 2Mg(OH), . MgCl2.5H2O and 9Mg(OH)2.MgCl2.6H2O formed at temperatures at and above 100 'C. Lower hydrates of the 3-form exist with 5 and 4 molecules of water, results which differ from those of Feitknecht and Held (1944), Feitknecht (1953), and Wehner (1933). It 1s not clear from the new results whether the lower hydrates for the 5-form hare 5 and 3 or 4 and 3 molecules of water, but the latter seems the more likely ; the results of Feitknecht and Held suggested the former. For the 2- and 9-forms a lower hydrate with 2 molecules of water exists. In addition all forms have an anhydrous phase with the crystal structure of Mg(OH)2. The stability of all phases under natural atmospheric conditions is discussed.