Molar enthalpies of dissolution of {ZnSO4 center dot 7H(2)O(s) + L-AA(s)} (in which AA are valine, methionine, histidine and threonine, respectively) in 100 cm(3) of 2 mol . dm(-3) HCl at T= (298.15 +/-0.001) K have been measured to be, Delta H-d(m,l)o = (94.588 +/- 0.025) kJ . mol(-1), (-70.619 +/- 0.051) kJ . mol(-1), -(13.129 +/- 0.013) kJ . mol(-1), and (72.001 +/- 1.602) kJ . mol(-1), respectively, by means of an isoperibol solution-reaction calorimeter. Molar enthalpies of dissolution of Zn(AA)SO4 center dot H2O(s) in 100 cm(3) of 2 mol . dm(-3) HCl at T= (298.15 +/- 0.001) K have been determined to be Delta H-d(m,2)o = -(46.118 +/- 0.055) kJ.mol(-1),(58.415 +/-0.046)kJ.mol(-1), -(11.271 +/- 0.012) kJ . mol(-1), and (6.9 +/- 0.8) kJ . mol(-1), respectively, by the calorimeter. The standard molar enthalpies of formation of the complexes Zn(AA)SO4 center dot H2O(s) have been derived to be, Delta H-f(m)o [Zn(Val)SO4 center dot H2O, s, 298.15 K] = -(1850.97 +/- 1.92) kJ . mol(-1), Delta H-f(m)o [Zn(Met)SO4 center dot - H2O, s, 298.15 K] = -(2069.304 +/- 0.743) kJ . mol(-1), Delta H-f(m)o [Zn(His)SO4 center dot H2O, s, 298.15 K] -(1827.61 +/-2.31) kJ . mol(-1) and Delta H-f(m)o[Zn(Thr)SO4 center dot H2O, s, 298.15 K] -(2111.9 +/- 3.40) kJ . mol(-1), by the combination of the experimental values of enthalpies of dissolution with some auxiliary thermodynamic data through a designed Hess thermochemical cycle on the basis of a supposed chemical reaction. (C) 2005 Elsevier Ltd. All rights reserved.