The molar enthalpies of reaction of metallic barium with 0.047 mol . dm(-3) HClO4 as well as the molar enthalpies of dissolution of BaCl2 in 1.01 mol . dm(-3) HCl and in water have been measured at T = 298.15 K in a sealed swinging calorimeter with an isothermal jacket. From these results the standard molar enthalpy of formation of the barium ion in an aqueous solution at infinite dilution, as well as the enthalpies of formation of barium chloride and barium perchlorate, are calculated to be: Delta(f)H(m)(0) (Ba2+ M, aq) = -(535.83 +/-1.25) kJ . mol(-1);. Delta(f)H(m)(0) (BaCl2, cr) = -(855.66 +/-1.28) kJ . mol(-1); and Delta(f)H(m)(0)(BaClO4, cr) = -(796.26 +/- 1.35) kJ . mol(-1). The results obtained are discussed and compared with previous experimental values.