The phase relationships of the nickel iodate-water system have been examined using thermal analyses (DSC), X-ray, IR and Raman spectroscopic methods, and especially high-temperature Raman and high-temperature X-ray experiments. The compounds established are Ni(IO3)(2) . 10H(2)O, which has been obtained for the first time (nu(OD) of matrix isolated HDO molecules in isotopically diluted samples : 2585, 2549, 2521, 2480, 2451, 2442, 2431, 2374, 2354, 2341 and 2295 cm(-1),95 K), alpha-Ni(IO3)(2) . 4H(2)O, beta-Ni(IO3)(2) . 4H(2)O(2443 and 2378 cm(-1), 95 K), Ni(IO3)(2) . 2H(2)O(2515 and 2427 cm(-1), 95 K), alpha-Ni(IO3)(2), and beta-Ni(IO3)(2) The IO stretching modes of nickel iodates resemble those of other solid iodates, whereas the bending modes are reinforced by up to 100 cm(-1), e.g. for alpha-Ni(IO3)(2) With nu(2) = 452 and 463 cm(-1), Ni(IO3)(2) . 10H(2)O (space group P (1) over bar, unit cell dimensions : a = 650.02(12), b = 1098.54(22), c = 1169.21(19)pm, alpha = 105.654 (11)(0), beta = 103.856(11)(0) and gamma = 101.515(11)(0))is isostructural to the respective magnesium compound. Decomposition and phase transition occur as follows : Ni(IO3)(2) . 10H(2)O (314 K, DSC)-->beta-Ni(IO3)(2) . 4H(2)O(421 K)-->Ni(IO3)(2) . 2H(2)O(431 K)-->alpha-Ni(IO3)(2)(745K)-->beta-Ni(IO3)(2)(790 K)-->NiO+I2O2 or alpha-Ni(IO3)(2) . 4H(2)O(384K)-->Ni(IO3)(2) . 2H(2)O-->, etc. Decomposition to the oxide instead of the iodide is favoured by the high stability of transition metal oxides.