This study has investigated the hydrotreatment of bio-oil (derived from the fast pyrolysis of mallee woody biomass) in a batch reactor under 10 MPa pressure with Pd/C catalyst at temperatures between 150 degrees C and 300 degrees C. Our results indicate that the chemical fractions, coking tendency as well as the aromatic structures are highly influenced by the hydrotreating conditions such as temperature and time. The repolymerisation surpasses the hydrogenation of bio-oil at the low hydrotreating temperatures (e g. 150-200 degrees C) and short hydrotreating durations (e. g. < 3 h). On the contrary, high hydrotreating temperatures (e.g. 250-300 degrees C) and long reaction durations (e. g. 6-12 h) can effectively convert the heavier fractions into lighter fractions, and thus further reduce the coking tendency of the hydrotreated products. However, these harsh operational conditions cannot decrease the number of large aromatic ring systems. Most importantly, it is found that the combination of esterification and hydrotreatment can produce more stable bio-oil with lower coking tendency and less large aromatic ring systems than the direct hydrotreatment of bio-oil. (C) 2013 Elsevier Ltd. All rights reserved.