Effect of hydrogen transfer from the pressurized gas or solvent upon liquefaction (with elemental sulfur as an accelerator at 420 degrees C for 1 h) of cobalt exchanged Adaro coal was discussed. Further, catalytic effect of exchanged cobalt upon liquefaction of cobalt exchanged coal was also discussed in comparison with the results after liquefaction of demineralized coal or complex [(C5H5)(2)Co or Co-2(CO)(8)] added demineralized coal. The amounts of transferred hydrogen from tetralin and the pressurized hydrogen gas upon liquefaction of cobalt exchanged coal with the pressurized hydrogen gas plus any solvent (tetralin, decalin or naphthalene) were almost equal to those of complex added demineralized coal. However, upgrading of cobalt exchanged coal occurred preferentially in comparison with that of complex added coal. In particular, the yield of residue was only 0.3% after liquefaction of cobalt exchanged coal with tetralin plus the pressurized hydrogen gas. Accordingly, liquefaction of cobalt exchanged coal was enhanced significantly by the following two reasons. It was considered that cobalt dispersed finely into the coal macromolecules with attached carboxyl groups irrespective of the size of coal molecule, and hydrogen shuttling through solvent was enhanced by catalytic effect of the dispersed cobalt. (C) 2010 Elsevier Ltd. All rights reserved.