The solid-state coordination reactions: 2HQ(s) + Co(Ac)(2).4H(2)O(s) --> Co(HQ)(2).2H(2)O(s) + 2HAc(1) + 2H(2)O(1) 2HQ(s) + Zn(Ac)(2).2H(2)O(s) --> Zn(HQ)(2).H2O(s) + 2HAc(1) + H2O(1) have been studied by solution calorimetry. The molar dissolution enthalpies of the reactants and the products in a certain solvent, 4 mol l(-1) HCl, of this solid-solid coordination reaction have been measured using an isoperibol calorimeter. From the results and other auxiliary quantities, the standard molar formation enthalpies of [Co(HQ)(2).2H(2)O(s), 298.15 K], [Zn(HQ)(2).H2O(s), 298.15 K] and have been determined to be Delta(f)H(m)(Theta) (Co(HQ)(2).2H(2)O(S), 298.15 K) = -766.9 +/- 3.0 kJ mol(-1), Q)(2).H2O(s), 298.15 K) = -580.7 +/- 3.0 kJ mol(-1).