Pine and poplar woods were liquefied using ethanol and phenol mixtures in the presence methanesulfonic acid (MSA) as catalyst, and the effects of the liquefaction variables were investigated. The woods were almost completely liquefied by replacing 10-13.3% of the total liquefying reagent with phenol. The optimum amounts of phenol for liquefaction depended on the lignin content of the wood and bark samples. A small amount of water addition (13.3% of the total liquefying reagent) was slightly effective for pine wood liquefaction, but it was not effective for poplar wood. To minimize the residue content, the amount of MSA was about 3.5 mmol for both woods, and a liquefaction temperature of at least 170℃ was required. The liquefaction follows a pseudo-first-order reaction in the early stage; it proceeded rapidly within 20 min for poplar wood and 30 min for pine wood at 180℃. The organic sulfonic acids, p-toluenesulfonic acid (PTSA) and MSA, and the mineral acid, sulfuric acid, were very effective acid catalysts for this liquefaction system, but hydrochloric acid was not. We found that lower alcohols other than ethanol could also be used successfully as liquefying solvents, but the polyhydric alcohols were less effective for the liquefaction.