To increase the understanding of the reaction behavior of heavy hydrocarbons in the presence of sub- and supercritical water, upgrading of residual oil was applied in a batch reactor at temperatures of 653 to 713 K and water densities of 0.05 to 0.20 g/cm(3). It is confirmed that upgrading of residual oil in sub- and supercritical water is still dominated by the free radical mechanism based thermal cracking. The ion mechanism based hydrolysis only has an extremely limited influence on the upgrading performance. With the increase in water density, the upgrading system may evolve from a partially miscible two-phase structure to a pseudo single-phase structure in which asphaltenes are highly dispersed in the continuous water phase. Prompt diffusion of aromatic radicals from asphaltenes into the water phase both depresses the coke formation and improves the liquid product distribution. (C) 2012 Elsevier B.V. All rights reserved.