The phase transitions, thermal stabilities and crystal structures of strontium iodate, barium iodate, calcium iodate, zinc iodate and cadmium iodate have been investigated by means of X-ray powder diffraction and thermal analyses. The possibilities of formation of complex compound between divalent iodates have been studied. The stability of hydrated divalent iodates is expected to be related to the electronegativity of the corresponding cations.