Fenuron [C6H5-NH-CO-NH(CH3)(2) I oxidation in hydrothermal conditions was examined in the temperature range between 200 and 540 degrees C at 25 MPa with residence times of 20-410 s. Under these conditions, the chemical oxygen demand reduction ranged between 70% (at 200 degrees C) and 99.92% (at 540 degrees C). Nitrogen atoms of the fenuron molecule were first transformed into ammoniacal nitrogen, nitrate ions and nitro groups of nitrobenzene. Then, they were converted into molecular nitrogen by a redox process between ammoniacal nitrogen and oxidized forms of nitrogen (NO3-, -NO2). At 540 degrees C and 25 MPa for a residence time of 39 s, fenuron was totally degraded into water, carbon dioxide and gas nitrogen, which are acceptable end products for the environment. (C) 2000 Elsevier Science B.V. All rights reserved.