The solubilities of potassium hydroxide and dibasic potassium phosphate in supercritical water solutions have been measured at temperatures from 400 to 525 degrees C and pressures from 22.1 to 32.0 MPa. The solubility of potassium hydroxide varied from 61 mg/kg at 525 degrees C and 22.1 MPa to 594 mg/kg at 450 degrees C and 30.4 MPa. The solubility of dibasic potassium phosphate varied from 2 mg/kg at 450 degrees C and 26.8 MPa to 416 mg/kg at 400 degrees C and 27.1 MPa. The solubility was found to vary directly with pressure and inversely with temperature. Results were fit to a semiempirical solvation model to yield predictive equations for solubility as a function of water density.