Bio-oil derived via slow pyrolysis process of two indigenous Australian tree species, red gum (Eucalyptus camaldulensis) from the basin of Murray, Victoria, and blue gum (Eucalyptus globulus) wood from the region of Mount Gambier, South Australia was blended with ethanol and burned in a circular jet spray at atmospheric pressure. Bio-oil flames were shorter, wider and brighter than diesel fuel flames at the same conditions. Adding of flammable polar additives (e.g. ethanol) to bio-oil improved some of the undesired properties of the fuel such as poor atomisation, low calorific value, and high NO, emission from the flame. Nevertheless, adding of ethanol should be carried out with caution since it leads to a reduction of the heat flux from the flame. Changing the concentration of flammable polar additives in bio-oil can be an optimising factor in achieving the proper balance between the best spray formation and the maximal heat flux from the flame. (c) 2005 Elsevier Ltd. All rights reserved.