Upgrading of bitumen was examined in supercritical water (SCW) and its mixtures with hydrogen and carbon dioxide using semibatch reactors. Coke formation became prominent above 673 K in SCW and 693 K for SCW + H-2 + CO2. Coke yield in SCW was higher than that in SCW + H-2 + CO2, implying that coke formation is suppressed in SCW + H-2 + CO2. The asphaltenes obtained with SCW + H-2 + CO2 had higher H/C atomic ratios than those of the raw asphaltene or asphaltene obtained with SCAN. The average molecular structures of raw asphaltene, as well as asphaltene in SCW and in SCAN + H-2 + CO2, were estimated. From the properties of asphaltene, repolymerization during decomposition of asphaltene was probably inhibited in SCW + H-2 + CO2. Furthermore, the oil formed in SCW + H-2 + CO2 was removed from the reactor more easily than that in SCW. The removal of oil from the reactor and the hydrogenation of asphaltene through reverse water-gas-shift reaction probably suppressed coke formation in SCW + H-2 + CO2.