The standard molar enthalpy of formation of the yttrium ion in aqueous solution at infinite dilution at T = 298.15 K was determined by two independent ways by solution calorimetry and was found to be - (733.51 +/- 2.07) kJ . mol(-1). During the experiment. the molar enthalpies of reaction of metallic yttrium with 1.07 mol . dm(-3) HCl and 0.88 mol . dm(-3) HClO4 as well as the molar enthalpies of dissolution of YCl3 in 1.05 mol . dm(-3) HCl and in water were measured in a sealed swinging calorimeter equipped with an isothermal jacket. This result is discussed and compared with previous experimental values.