This study reports the effects of a natural calcium borate mineral, colemanite, on the hydrothermal liquefaction of beech wood biomass. Hydrothermal liquefaction experiments were performed at 250, 300 and 350 degrees C with and without colemanite. The highest light bio-oil yield (11.1 wt%) and the highest heavy bio-oil yield (29.8 wt%) were obtained at 300 degrees C with colemanite. The total bio-oil yields were 22 wt% and 41 wt% ca. at 300 degrees C without and with colemanite, respectively. The highest heating values were obtained from the hydrothermal liquefaction of biomass using colemanite at 350 degrees C: 23.81 MJ/kg for LBO and 27.53 MJ/kg for HBO. Most of the identified compounds in the light bio-oils were phenols. The light bio-oil obtained without the catalyst at 300 degrees C contained furfurals. However, these compounds were not observed in the run with colemanite at the same temperature. The heavy bio-oils from the runs both without and with colemanite are composed of phenols, aldehydes, ketones, acids, and benzene derivatives. 4-Methyl-3-penten-2-one was the major compound in the heavy bio-oils produced in the catalytic runs. (C) 2012 Elsevier B.V. All rights reserved.