Taiheiyo coals were extracted with supercritical toluene (SC-toluene) (653 K, 20 MPa), supercritical water (SCW) (653 K, 35 MPa) and formic acid (HCOOH)-SCW mixed solvents (653 K, 35 MPa) using a semi-batch type system. The results clearly indicate that the coal conversion (= 1 - weight of residual coal (daf)/weight of loaded coal (daf)) and the liquid yield (= weight of liquid extracts/weight of loaded coal) in HCOOH-SCW were higher than those in SCW and SC-toluene. A considerable portion of coal is thus converted into light oils probably through hydrolysis and hydrogenation in HCOOH-SCW. We conducted another series of experiments for direct in situ observation of coal conversion in SCW and HCOOH-SCW using a diamond anvil cell (DAC). In HCOOH-SCW, the effective and rapid hydrogenation of coal with HCOOH occurs mainly at the early stage of the reaction. (C) 2000 Elsevier Science Ltd. All rights reserved.