Magnesium phthalate dihydrate was synthesized by rheological phase reaction. The crystal structure was determined by XRD. Its thermal decomposition mechanism in N-2 was studied by TG DTA. The thermal decomposition products were characterized by XRD, GC-MS. The thermal decomposition of magnesium phthalate dihydrate in N-2 proceeded in two stages; it first lost two crystal water to form anhydrate salt, then magnesium phthalate decomposed further, the last product was MgO. The organic compounds are mainly benzophenone and anthraquinone. (c) 2004 Elsevier B.V. All rights reserved.