Supercritical water and supercritical water partial oxidation treatments were applied to the upgrading of asphalt. Asphalt was converted at 613-673 K, 0-0.5 g/cm(3) water density under argon or air atmosphere. Under an argon atmosphere and 0.5 g/cm(3) water density, both the asphaltene conversion and desulfurization increased with increasing temperature. At 673 K, the asphaltene conversion and the yield of CO2 increased with an increasing water density. Water apparently participated in the reaction and its hydrogen was used for capping the free radicals generated during the upgrading of asphalt resulting in an increased yield of maltene. Under an air atmosphere at 673 K, asphaltene conversion was lower but desulfurization was higher than those obtained in an argon atmosphere. (C) 2003 Elsevier Science Ltd. All rights reserved.