Magnesium chloride hydrate can be dehydrated to some extent by heating. However, it is not possible to fully dehydrate magnesium chloride by heating in air because of hydrolytic decomposition. Accordingly, the dehydration should be carried out in hydrogen chloride gas atmosphere. However, this process causes many problems, including HCl gas storage and corrosive nature, consuming a large amount of HCl gas. Consequently, many alternative approaches have been tried for the production of anhydrous magnesium chloride. In this work, we proposed a process to prepare anhydrous magnesium chloride using ammonium magnesium chloride hexahydrate (NH4MgCl3 center dot 6H(2)O). From this work, it was confirmed that anhydrous magnesium chloride can be produced by the dehydration of magnesium chloride hydrate in HCl gas atmosphere, and found that anhydrous magnesium chloride can be prepared from ammonium magnesium chloride hexahydrate at the reaction temperature range 50-400 degrees C and a reaction time of 30 min even in air atmosphere. (C) 2010 The Society of Powder Technology Japan. Published by Elsevier B.V. and The Society of Powder Technology Japan. All rights reserved.