A new isothermal flow calorimeter for measuring heats of mixing of two fluids at high temperatures is described. The primary use of the calorimeter is for the study of reactions in high-temperature aqueous solutions, The calorimeter was designed for operation to 450 degrees C and 50 MPa for both exothermic and endothermic processes. Platinum-rhodium alloy tubing was used in the calorimeter to resist corrosion by electrolyte solutions under extreme experimental conditions. Preheating of the solutions was achieved by submerging coils of the tubing in a salt bath, An automatically controlled pulsed heater and a constant heat-leak path were used to maintain the reaction zone at a constant temperature, An all-liquid pressure control system was added for operations at higher pressures, Energy effects from 0.0025 to 2 J s(-1) can be measured to a precision of +/- 1.5% or +/- 0.0005 J s(-1), whichever is greater. The calorimeter was tested by measuring heats of dilution of aqueous solutions of NaOH at 300 degrees C (9.3 MPa) and of NaCl at 300 degrees C (9.3 MPa) and 350 degrees C (17.6 MPa).