The standard molar enthalpy of solution of double salts, K 2 Mg(IO 3 ) 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: Δ f H m Θ (K 2 Mg(IO 3 ) 4 .2H 2 O,s,298.2K=-2510.69kJmol - 1 . The standard free energy {Δ f G m Θ (K 2 Mg(IO 3 ) 4 .2H 2 O,s,298.2K)=-2061.72 JKmol - 1 } and the absolute entropy (S m Θ (K 2 Mg(IO 3 ) 4 .2H 2 O,s,298.2K)=258.36JK - 1 mol - 1 were also calculated.