Bio-oils are usually dark brown, free-flowing liquids having a distinctive smoky odor. The physical properties of bio-oils are described in several publications [14,28]. The different physical properties of bio-oils result from the chemical composition of the oils, which is significantly different from that of petroleumderived oils. Bio-oil is a complex mixture of several hundreds of organic compounds, mainly including acids, alcohols, aldehydes, esters, ketones, phenols, and lignin-derived oligomers. Some of these compounds are directly related to the undesirable properties of bio-oil. Basic data for liquefied bio-oils from swine manure, pyrolysis bio-oil from wood and conventional petroleum fuels are compared in Table 3.