DTA, TG and DTG curves were recorded for ground mixtures of hydrated MgCl2 and MgBr2 with NaCl, NaBr, KCl, KBr, CsCl and CsBr. Infrared spectra of the ground mixtures were recorded after they had been heated to several temperatures. Substitutional solid solutions of hydrated magnesium in potassium and cesium salts were formed during grinding of the salt mixtures. However, a solid solution of MgCl2 is always obtained with KCl and CsCl, as a result of grinding the alkali chloride either with MgCl2 or with MgBr2. Also, a solid solution of MgBr2 is always obtained with KBr and CsBr, either from MgCl2 or from MgBr2. Cesium halides form stable solid solutions with the magnesium halides, and thermal hydrolysis of the hydrated magnesium is prevented. The solid solutions formed with potassium halides are less stable than those formed with cesium salts and hydrated Mg is thermally hydrolyzed. The temperature at which this hydrolysis occurs in the presence of potassium halide is above 300 °C, whereas in its absence this reaction occurs at temperatures below 250 °C. No hydrated solid solution of Mg is formed with sodium halides during the grinding process and the thermal behavior of magnesium chloride or bromide in sodium chloride or bromide, respectively, is similar to the thermal behavior of the pure magnesium salt. The thermal behavior of MgBr2 in NaCl is similar to that of pure MgCl2 but the thermal behavior of MgCl2 ground with NaBr shows some similarities with each of the two pure magnesium salts, the chloride and the bromide.