The solubility of a nickel ferrite, NixFe3-xO4, as a model substance for deposits forming on the cladding of nuclear fuel elements of pressurized water reactors is determined in aqueous solutions of primary coolant composition at different pH and hydrogen concentrations, in the temperature region from 230 to 330°C. The temperature coefficient of solubility changes from negative to positive at about pH 7 with increasing pH. The data agree reasonably well with those of Sweeton and Baes for magnetite, considering the lower iron activity in the nickel ferrite. The solubility of nickel is generally lower than that which corresponds to a congruent solution and goes through a minimum near pH 7.4. The iron solubility appears to depend on the one-third power of the hydrogen concentration in solution in the region from 20 to 100 cm3 H2/kg H2O, as expected. Preliminary results, however, indicate that at lower hydrogen concentrations, the dependence on the hydrogen concentration can decrease, possibly due to the formation of cation vacancies.