Knowledge of the phase equilibria and solubility of inorganic salts is crucial to improving the handling of salt precipitation in supercritical water oxidation (SCWO) processes. However, there is little such data in the literature, especially at 30 MPa, the pressure at which the continuous SCWO processes of the CEA (the French atomic energy commission) operate. In this study, the solubility of Na2CO3 and K3PO4 was determined under near-critical and supercritical conditions (320 degrees C < T < 500 degrees C, P = 30 MPa) by high pressure differential scanning calorimetry or by measuring the conductivity of samples taken from a high-pressure vessel. This approach enabled different phase changes to be detected for the two salts: direct solid precipitation from the homogeneous fluid phase for Na2CO3, and the appearance of a gas-liquid equilibrium before precipitation for K3PO4. On the whole, the results obtained are in agreement with Valyashko's classification of binary water/salt systems.