The solubilities of lead oxide (PbO) and copper oxide (CuO) in subcritical and supercritical water weremeasured at temperatures from 250 °C to 500 °C and pressures from 26 MPa to 34 MPa, in a flow-typeapparatus. The solubility of lead oxide varied from 351 μmol/kg H2O at 424.9 °C and 25.9 MPa to 4406μmol/kg H2O at 350.3 °C and 30.2 MPa. The solubility of copper oxide varied from 1.3 μmol/kg H2O at449.8 °C and 28.0 MPa to 8.0 μmol/kg H2O at 324.9 °C and 28.1 MPa. A hydration reaction model wasapplied to correlate the data.