Olive husk, hazelnut shell, spruce wood, and beech wood samples were pyrolyzed to obtain pyrolysis oil. Liquid fuel produced from biomass is pyrolysis oil, formed by rapidly heating biomass to drive off the organic volatile components, then condensing the fuel vapors into a fuel oil. Thermal depolymerization and decomposition of biomass structural components, such as cellulose, hemicelluloses, lignin form liquids, and gas products as well as a solid residue of charcoal were studied. The structural components of the biomass samples mainly affect pyrolytic degradation products. A reaction mechanism is proposed which describes a possible reaction route for the formation of the characteristic compounds found in the pyrolysis oils. A large fraction of the oil is the phenolic fraction, consisting of relatively small amounts of phenol, eugenol, cresols, and xylenols and much larger quantities of alkylated polyphenols. The pyrolysis oil contents and the higher heating values of the oils vary from 32.1 to 49.3% of dry and ash-free basis and from 22.5 to 25.7 MJ/kg, respectively.