The scaling-up of hydrothermal oxidation process is performed by the use of simulation tools, which require an important experimental support. In this way, we have proposed a new experimental approach to determine the global reaction heats of hydrothermal oxidation reactions. A quasi-adiabatic tubular reactor was developed in order to follow the temperature profiles along the reactor during the oxidation reaction. From the experimental temperature profiles, a model of the reactor thermal behavior was proposed and gives access to the global reaction heats. The experimental setup and model have been validated for acetic acid oxidation in supercritical water with hydrogen peroxide as oxidant. The global reaction heat of acetic acid hydrothermal oxidation (P = 25 MPa, 400 less than or equal to T less than or equal to 570 degreesC) has been found equal to - 925 kJ mol(-1). (C) 2001 Elsevier Science B.V. All rights reserved.