The phase transitions of Mg(IO3)2·4H2O crystals in the heating process have been investigated by means of thermal analyses and X-ray diffraction at high and room temperatures. The results may be exhibited in the following diagram:no thermal effect observed on beating or coolingThe crystal structure of Mg(IO3)2·4H2O has been determined by means of the method of single crystal diffraction, it has been found that the space group of Mg(IO3)2·4H2O is P2 or Pm. There are two formula units per unit cell. Lattice parameters are: a=8.307?, b=6.627?, c=8.541?, β=100°5′. The crystal structure of α-Mg(IO3)2 has been determined by X-ray powder method. The unit cell of α-Mg(IO3)2 is hexagonal, with a=5.4777?, c=5.1282?, c/a= 0.9362 at 14℃. There is one formula unit per unit cell. The space group is P63. 60II are situated at the 6(c) positions, 2IV are situated at 2(b) positions, and Mg++-ion is disorderly distributed at half of 2(a) positions, the atomic parameters being Xc=0.096, Yc=0.344, Zc=0.162, Zb=0, Za=0.981. The crystal structure of α-Mg(IO3)2 is isomorphic with a-LiIO3. α-Mg(IO3)2 is different from a-LiIO3 only in that the Mg++-ion is disorderly distributed at one-half of 2(a) positions and the other half is vacant.