The standard molar enthalpy of solution of double salts, K2Mg(IO3)(4). 2H(2)O(s), in water has been measured by solution calorimetry. From that value, combined with auxiliary values, the standard molar formation enthalpy has been derived: Delta(f)H(m)(-) (K2Mg(IO3)(4). 2H(2)O,s,298.2 K = -2510.69 kJ mol(-1). The standard free energy {Delta(f)G(m)(-) (K2Mg(IO3)(4). 2H(2)O,s,298.2 K) = -2061.72 J K mol(-1)} and the absolute entropy (S-m(-)(K2Mg(IO3)(4). 2H(2)O,s,298.2 K)= 258.36 J K-1 mol(-1) were also calculated.