Pine (Pinus densiflora) and poplar (Populus alba × glandulosa) woods are liquefied by dioxanel polar solvent mixtures in the presence of mineral or organic sulfonic acid catalysts. Mixing 6.7- 40% of water with dioxane remarkably improved the liquefaction; this observation may be due to the hydrolysis effect and enhanced solubility of the liquefied material in the solvent. A similar improvement in the liquefaction also was observed when using other highly polar solvents, such as methanol, ethanol, and ethylene glycol (EG). Organic sulfonic acids, e.g., p-toluenesulfonic acid (PTSA), methanesulfonic acid (MSA), and naphthalenesulfonic acid (NSA), were slightly more effective as catalysts for the liquefaction than were mineral acids. More than 95% liquefaction was achieved within 30 min at 160 - 170℃, even when liquifying softwood.