A new double salt K2Mg(IO3)(4) . 2H(2)O has been synthesized. Its thermal and calorimetric behaviors were investigated by the DTA, TG and DSC methods. A mechanism of thermal decomposition has been suggested. The Delta(f)H(298)(O), was calculated from the recorded Delta H-0 of the DSC curve. The data from the TG curves were processed by the methods of formal kinetics involving the calculation of the kinetic parameters E-* and Z, and the type of the kinetic equation determined. The solubility of K2Mg(IO3)(4) . 2H(2)O in water was investigated in the 20-50 degrees C temperature interval. The value of Delta(dissol)H(298)(0) was calculated on the basis of the solubility data, whence the value of Delta(f)H(298)(0) of the double salt was calculated. The values of Delta(f)H(298)(O), determined by the two methods, were compared.