Measurements of the solubility of crystalline cupric oxide were made in liquid water and steam at controlled pH conditions from 25.0 degrees C to 400 degrees C using flow-through packed columns and batch sampling techniques. Only partial agreement with literature values was evident for CuO(cr) solubility in liquid water leading to large differences in the first two hydrolysis constants of Cu2+ compared to those from numerous potentiometric studies. The concentrations of NaOH employed in this study were too low to form measureable amounts of CudOH(4)(2-) in solution and the Cu(OH)(2) (aq) species could only be detected in the regression processes at <= 50 degrees C. However, the presence of CuOH+ and CudOH(3)(-) was established unambiguously from 25 degrees C to 350 degrees C. The solubility of CuO(cr) was found to be higher than that of Cu2O(cr) at ambient conditions, except at very high pH, whereas at 350 degrees C Cu2O(cr) was the more soluble phase in oxygen-free water. General agreement was found between the measured solubilities in steam under mainly sub-critical conditions and those from a number of previous studies in super-critical water, albeit within the inherently large scatter. The solubility CuO(cr) in steam was modelled as a function of steam density. Only the neutrally charged molecule CudOH(2)(aq) is thought to partition between the phases in the absence of significant concentrations of complexing anions. (C) 2017 Elsevier Ltd.