Current global resources of fossil fuels are gradually depleting and the energy crisis induces increasing concerns on the research of new effective substitution of these fossil fuels by renewable energy, especially bio-fuels from biomass such as bio-oils. However, bio-oils, generally originated from the pyrolysis of biomass, contain a great deal of carboxylic acids such as acetic acid and these acids can easily decrease the stability and the quality of oil. Meanwhile, these acids are highly corrosive to reaction equipments. Bio-oil could be upgraded before its utilization in the feedstocks of fuels and chemicals. In this work, the removing of these carboxylic acids was investigated by esterification in supercritical ethanol. The effects of reaction temperature, the ratio of ethanol to bio-oil, and reaction time on the conversion of acids were studied as well as the addition of external acid such as H2SO4, H3PO4 or zeolite. The results showed that carboxylic acids in crude bio-oil easily esterified with ethanol in the supercritical system. More ethyl acetate was formed at higher volume ratio of ethanol to bio-oil and 100% of the selectivity was achieved at the volume ratio of 5:1 after 2 h reaction, whereas more side reactions were present in lower or higher ratio of ethanol to bio-oil. The addition of external acid decreased distinctly the formation of esters, indicating that these carboxylic acids could be effectively removed under the acidic system arising from the internal ionization of ethanol. These would be very useful in the upgrading of bio-oil into high quality fuels in the future biorefinery. (C) 2015 Elsevier Ltd. All rights reserved.