The standard molar enthalpies of dissolution of mixtures K2SO4:2MSO(4) and double salts K2M2(SO4)(3) (where M = Cd2+ Zn2+ or Mg2+) in aqueous 3 M HNO3 have been measured by using an isoperibol reaction calorimeter at 298.2 K. From the values obtained together with auxiliary data and by using appropriate thermochemical cycles, the standard molar enthalpies of formation of K2M2(SO4)(3) at 298.2 K were hence derived. Delta H-f(m)theta[K2Cd2(SO4)(3(s))] = -3305.52 +/- 0.17 kT mol(-1) Delta H-f(m)theta[K2Zn2(SO4)(3(s))] = -3406.85 +/- 0.23 kT mol(-1) Delta H-f(m)theta[K2Mg2(SO4)(3(s))] = -3953.07 +/- 0.25 kJ mol(-1)