In this paper, we have investigated for the first time heavy oil conversion under the conditions of activated carbon (AC) oxidation by supercritical water-oxygen (SCW/O-2) fluid at temperature 723 K, pressure 30 MPa, constant flow rate of water and oil (6 and 4g/min, respectively), and variable flow rate of oxygen (0-3.5 g/min). The experiment was performed by supplying the oil (from the top) and SCW/O-2 fluid (from the bottom) into a vertically located tubular reactor packed with AC. The obtained time dependences of the reactor wall temperature and the power of resistance heaters allowed revealing that due to heat generation during the combustion of AC and oil high-molecular components in SCW/O-2 fluid the conversion process occurs in autothermal mode. The overall yield of volatile combustibles and liquid products of oil conversion amounted to 24.0 and 59.5 wt.%, respectively. It is shown that the involvement of water in redox reactions with AC and oil high-molecular components accumulated in the AC bed, contributes to the enrichment of conversion products with hydrogen (the portion of H-2 and CH4 in the volatile combustible products is > 60 mol.%). We detected a significant decline in the fractions of asphaltene and resin, as well as the content of vanadium (V) and nickel (Ni) in the liquid products compared to crude oil due to their accumulation in the AC bed. It follows from the results of IR and H-1 NMR spectroscopy that a significant increase in the content of aromatic compounds in the liquid products occurs only at the increase in oxygen flow rate up to 3.5 g/min. (C) 2017 Elsevier B.V. All rights reserved.