The decomposition Of MgCl2.6NH3 and MgCl2.2C(2)H(6)O(2).2NH(3) in static air was investigated by DTA-TG, XRD and chemical analysis from room temperature to 700degreesC. The results indicated that MgCl2.6NH(3) took three steps to decompose to anhydrous MgCl2 below 450degreesC and at higher temperatures resulted in the oxidation of anhydrous MgCl2. This theoretically confirms the validity of the operation in which Ar or N-2 is used in manufacturing anhydrous MgCl2 at 650degreesC. MgO2.2C(2)H(6)O(2).2NH(3) experienced a complicated process of decomposition, Mg(OH)Cl and carbon were obtained at 300degreesC and MgO at 530degreesC, respectively, which may decrease the quality of anhydrous MgCl2. (C) 2003 Elsevier B.V. All rights reserved.