Upgrading of bitumen was performed in a semi-batch type reactor in supercritical water (SCW) and in mixtures of SCW with CO or H-2 from 673 to 723 K at 30 MPa. The extracted oil yield was in the order SCW + CO >SCW + H-2 > SCW at 673 K. CO and H-2 facilitated the extraction of the oil from inside the reactor to the outside the reactor. The coke yield in SCW + CO was lower than the yields in SCW and SCW + H-2, indicating that hydrogenation of bitumen through the water-gas shift reaction probably occurred. Furthermore, the lightest extracted oil was obtained at 693 K in SCW + CO. The distribution of oil between the water-rich phase and oil-rich phase in the reactor governed the stability of the coke precursor in the oil-rich phase and the temperature determined the oil decomposition rate and the coke formation rate. The optimal upgrading condition was dependent on these factors. (C) 2015 Elsevier B.V. All rights reserved.