A 0.1 M potassium phosphate (K2HPO4) solution was reacted in a flow-through cell pressurized to 22 MPa. Reduced light transmission through the cell windows was observed at a setpoint temperature >= 400 degrees C, along with a decrease in effluent conductivity, but with no effect on flow. These observations suggest solution separation at similar to 360 degrees C, with accumulation of a salt-concentrated liquid in the cell body and transition of a dilute liquid to a supercritical fluid at temperature >374 degrees C. High-pressure differential scanning calorimetry experiments confirm an onset temperature of 354 degrees C with an endothermic transition at 377 degrees C and 22 MPa. For apparent density, rho = 150-500 kg/m(3), the average transition temperature for 0.1 M solutions, 375 +/- 5 degrees C, is slightly elevated relative to that of water at 371 +/- 4 degrees C. Highest deviation for 1.0 M solutions, 365 +/- 15 degrees C, is attributed to increased K2HPO4 hydrolysis and polymerization reactions. (C) 2011 Elsevier B.V. All rights reserved.