The aqueous solubilities of hellyerite, NiCO3.6H(2)O, and gaspeite, NiCO3, were. thermodynamically analyzed to obtain the respective standard molar quantities of formation Delta(f)G(m)(circle minus) and Delta(f)H(m)(circle minus). For the molar entropy, S-m(circle minus) (NiCO3.6H(2)O), no direct calorimetric measurements are available, so it was derived from solubility data also. Solubility constants extrapolated to infinite dilution using the specific ion-interaction theory and the Pitzer equations, respectively, agreed with each other within the experimental uncertainty. All calculations were carried out with the ChemSage optimizer and based on auxiliary quantities which were either recommended by CODATA or recently critically evaluated. The following set of thermodynamic data for NiCO3.H2O and NiCO3 (numerical values of the latter in brackets) were obtained at T = 298.15 K: solubility constant, log* K-ps0(0) = [Ni2+] x p(CO2) x [H+](-2) = 10.64 +/- 0.10 (7.12 +/- 0.18); standard Gibbs energy of formation, Delta(f)G(m)(circle minus) (kJ mol(-1)) = -2039.2 +/- 1.1 (-636.5 +/- 1.4); standard enthalpy of formation, Delta(f)H(m)(circle minus) (kJ mol(-1)) = -2456.7 +/- 3.1 (-713.4 +/- 1.6); standard entropy, S-m(circle minus) (NiCO3 . 6H(2)O) (J mol(-1) K-1) = 343 +/- 10.