The solubility of NiO and CoO in high-temperature water is measured at temperatures between 373 and 523 K in a special batch autoclave system. Great importance has been given to deriving reliable thermodynamical data for the related ionic species at elevated temperatures. The experimental results for NiO seem to be consistent with the previously proposed free-energy data set for Ni2+. Because of the lack of experimental information on the free energy of formation (ΔG°f) for Co2+, the Criss-Cobble approach is used to calculate the values. The estimates are compared with the experimental results. It is concluded that the application of the Criss-Cobble procedure to Co2+ ions is not correct above 423 K. Based on the experiments, a new set of ΔG°f values for Co2+ is proposed.