The amount of wood residue is used as a measurement of the extent of wood liquefaction. Characterization of the residue from wood liquefaction provides a new approach to understand some fundamental aspects of the liquefaction reaction. Residues were characterized by wet chemical analyses, Fourier transform infrared (FTIR) spectroscopy, X-ray diffraction (XRD), and scanning electron microscopy (SEM). The Klason lignin content of the residues decreased, while the holocellulose and alpha-cellulose contents increased as the phenol to wood ratio (P/W) increased. A peak at 1735 cm(-1), which was attributed to the ester carbonyl group in xylan, disappeared in the FTIR spectra of the residues from liquefied wood under a sealed reaction system, indicating significantly different effects of atmospheric versus sealed liquefaction. The crystallinity index of the residues was higher than that of the untreated wood particles and slightly increased with an increase in the P/W ratio. The SEM images of the residues showed that the fiber bundles were reduced to small-sized bundles or even single fibers as the P/W ratio increased from 1/1 to 3/1, which indicated that the lignin in the middle lamella had been dissolved prior to the cellulose during liquefaction. (c) 2007 Wiley Periodicals, Inc.