Struvite, an important reaction product in magnesium ammonium phosphate cement systems, was synthesized in the laboratory. The elevated-temperature dehydration and then roomtemperature hydration characteristics of the dehydrated products were studied by thermogravimetric analyses and X-ray diffraction techniques. From isothermal experiments, struvite is found to be thermally unstable in air at temperatures above 50 C. Struvite can lose part or all of its ammonia and water molecules depending on the time and temperature of heat treatment, ultimately forming magnesium hydrogen phosphate. This decomposed product is X-ray amorphous and upon room-temperature rehydration can form struvite, unknown hydrates or newberyite, alone or in combination with each other, depending on the amount of ammonia left in the structure. However, when struvite is heated in excess water, it only loses its water of crystallization to form the monohydrate, dittmarite. Dittmarite is thermally more stable than struvite and like struvite also forms magnesium hydrogen phosphate on decomposition. At room temperature and in the presence of excess water, dittmarite can slowly transform to the hexahydrate, struvite. The consequence of structural similarities between struvite and dittmarite and conditions under which they may be present in cured cements are described.