In this communication we report calorimetric data for the standard state enthalpies of solution of alpha-Ba(OH)(2) in high dilution (10(-3) m) hydrochloric acid obtained from integral heats of solution measurements from temperatures of (333.55 to 516.64) K and extrapolated to 298.15 K. From previous studies in this laboratory on BaCl2(aq) and auxiliary literature data. the standard state thermodynamic functions for completely ionized HCl(aq) can be determined. These new data are in good agreement and confirm our previously reported results on HCl(aq) from ionic additivity. The enthalpy of formation of solid alpha-Ba(OH)(2) at temperature of 298.15 K of -939.38 kJ . mol(-1) can also be calculated from the present results. Values of the standard state heat capacity change for the ionization of water up to temperature of 523.15 K and at p(sat), were calculated from present results using the literature data for NaOH(aq) and NaCl(aq) obtained from high dilution calorimetric measurements. (C) 2009 Elsevier Ltd. All rights reserved.