The following reactions 2Zn(Ac)(2) . 2H(2)O (s) + H5IO6 (s) --> Zn2HIO6 . 1.25H(2)O (s) + 4HAc (1) + 2.75H(2)O (1) 2Cd(Ac)(2) . 2H(2)O (s) + HsIO(6) (s) --> Cd2HIO6 . 1.25H(2)O (s) + 4HAc (1) + 1.25H(2)O (1) 2Cu(Ac)(2) . H2O (s) + H5IO6 (s) --> Cu2HIO6 2H(2)O (s) + 4HAc (1) have been studied. The molar dissolution enthalpies of the reactants and products in a certain solvent (3 M l(-1) HNO3) of this three reaction have been measured using an isoperibol calorimeter at 298 K. From these values, combined with auxiliary values, the standard molar formation enthalpies of Zn2HIO6.1.25H(2)O (s), (298.2K), Cd2HIO6.1.25 H2O (s), (298.2K) and Cu2HIO6.2H(2)O (s), (298.2 K) have been determined to be Delta(f)H(m)(theta), (Zn2HIO6.1.25 H2O (s), 298.2 K) = -1342.11 kJ mol(-1) Delta(f)H(m)(theta) (Cd2HIO6.1.25H(2)O (s), 298.2 K) = -1405.49 kJ mol(-1) and Delta(f)H(m)(theta) (Cu(2)HIO(6)2H(2)O (s), 298.2 K) = -1226.08 kJ mol(-1)