A new isothermal how calorimeter for measuring heats of mixing of two fluid streams at high temperatures and pressures is described. The main advantages of the new calorimeter over earlier ones are simplification of the design making it easier to maintain the equipment, reduction of the temperature gradients so that baseline corrections are minimized, and ability to make reliable measurements above 350 degreesC. Immersion of the reaction vessel and associated tubing in a eutectic salt bath and the use of a thermoelectric cooler made it possible to maintain the reaction zone at a constant temperature. Energy effects from 2.5 to 2000 mW can be measured to a precision of +/-1.5% or +/-0.5 mW, whichever is greater. The relatively high uncertainty is due to the difficulties of operating at temperatures and pressures near and above the critical point of water. The calorimeter was tested by measuring the heat of mixing of sodium chloride solutions and water at 300 (9.3 MPa), 350 (17.6 MPa), 370 (24.7MPa), and 380 degreesC (24.7MPa). The results at 300 and 350 degreesC were in good agreement with our earlier results [Thermochim. Acta 285 (1996) 11] and were within 10% of earlier results by Busey at al. [J. Chem. Thermodyn. 16 (1984) 343]. No data are available in the literature at 370 and 380 degreesC. However, our results at these temperatures appear to be reasonable.