The mechanism of synergistic interaction between bitumen and coal in coprocessing was investigated in conjunction with hydrogen transfer between them. Two types of reaction systems were used in this work : the upgrading of bitumen and coal-derived oil, and the coprocessing of bitumen with either coal-derived oil or coal. Considerable retrogressive reaction was observed in the upgrading of bitumen alone at 450 degrees C but was effectively suppressed by the addition of either coal-derived oil or coal. These results strongly suggest that both coal-derived oil and coal act as good hydrogen donors or shuttlers in the coprocessing. Furthermore, their addition resulted in more production of light oil from bitumen. The conversion of coal into toluene solubles was influenced by the concentration of coal itself in the slurry feed, but the formation of distillable oil from coal seemed to be negligibly small.