An isothermal flow calorimeter suitable for measuring heats of mixing of two fluid streams which may or may not contain reactive solutes is described. The calorimeter proper was constructed of a platinum-10% rhodium alloy (tubing) and platinum-10% iridium alloy (mixing chamber) in order to obtain corrosion resistance to both strong acids and strong bases. Endothermic and exothermic processes can be studied from 323 to 398 K and from 0.1 to 20 MPa with the calorimeter. Energy effects from 0.0025 to 0.50 J sec(-1) are measured at a constant temperature and pressure through the use of an automatically controlled pulsed heater and a constant cooling Peltier device. The calorimeter was tested (pressures given in parentheses) by measuring the heat of mixing of ethanol and water at 348.15 K (1.52 MPa) and the heat of dilution of aqueous sodium chloride at 323.15 (0.34 MPa), 348.15 (0.34 MPa), 372.85 (6.6 MPa) and 398.15 K (1.03 MPa), and was found to produce data in good agreement with literature values.