Upgrading bitumen by converting asphaltene to maltene whilst suppressing coke formation is desirable though technically difficult. Mass transfer behavior under sub-and super critical water conditions was observed in an autoclave-type reactor revealing that the dissolution of asphaltene core attenuates coke formation and promotes the formation of upgraded products. On the basis of this new information, a continuous hydrothermal extraction method using near supercritical water in a column flow reactor was tested. The results at 300 degrees C, 3-6 MPa, and water flow of 3-10 g/min, showed higher degree of upgrading than in the autoclave reactor. Significant conversion of asphaltene to maltene with lighter maltene composition was obtained, while coke formation was completely suppressed. A continuous hydrothermal extraction method using a column flow reactor has thus been shown can solve the problem of incompatibility between enhancing the degree of upgrading of heavy oils and suppressing the formation of coke. In the proposed mechanism, low levels of asphaltene core extraction results in the formation of coke in the oil phase, whereas high levels of asphaltene core extraction results in the suppression of coke formation due to low concentrations of asphaltene core remaining in the oil phase, and the solvent properties that asphaltene core acquires at high concentrations in the water phase. (C) 2012 Elsevier Ltd. All rights reserved.