Hydrothermal liquefaction (HTL) of biomass to bio-oil under alkaline or neutral conditions has been widely reported in literature. However, there has been limited data available in literature on comparing HTL of biomass to bio-oil under acidic, neutral, and alkaline in terms of chemical compositions and yields by using the same reaction conditions and reactor. Using cellulose as a feedstock we conducted the comparative studies for pH = 3,7 and 14 at temperatures of 275-320 degrees C with reaction residence times of 0-30 min. Results showed that the chemical compositions of the bio-oils were different for acidic, neutral and alkaline conditions. Under acidic and neutral conditions. the main composition of HTL bio-oil was 5-(Hydroxymethyl)furfural (HMF). Under alkaline conditions, the main compounds became C2-5 carboxylic acids. For bio-oil yields, it was observed that high temperatures and long residence times had negative effects, regardless of the pH levels. However, the corresponding reaction mechanisms are different. Under acidic conditions, the decrease in the bio-oil yields was mainly caused by polymerization of 5-HMF to solids. Under neutral conditions, the bio-oil yields decreased because 5-HMF was converted to both solid and gaseous products. Under alkaline conditions, the bio-oil decomposed to gases through the formation of short chain acids and aldehydes. Therefore, although they were all referred to as HTL bio-oil in literature, they were formed by different reaction pathways and had different properties due to their different chemical compositions. Given these differences, different strategies are recommended in this study to further improve HTL of biomass to bio-oil. (C) 2011 Elsevier Ltd. All rights reserved.