In this study, noncatalytic and catalytic fast pyrolysis of forest thinnings was carried out in a bench-scale bubbling fluidized bed reactor using two commercial ZSM-5 catalysts. After the initial comparison experiments at 475 degrees C, the catalyst which showed a clearly higher activity was further tested at reaction temperatures of 400 and 550 degrees C. The yield of organic liquids, which was 39 wt % in the noncatalytic experiment, decreased to 14-24 wt % depending on the reaction temperature. Varying the reaction temperature while using the ZSM-5 catalyst also caused significant changes in the elemental composition, viscosity, and pH, as well as the chemical composition of bio-oil. In general, the use of the catalyst resulted in a less viscous bio-oil with an oxygen content as low as 19 wt %. The catalytically produced bio-oil contained less carbohydrate degradation products and more aromatic compounds. Based on the chemical characterization results (C-13 nuclear magnetic resonance spectroscopy, gas chromatography-flame ionization detector, gas chromatography mass spectrometry, and solvent fractionation) obtained in this study, a new supplementary procedure for the existing solvent fractionation method was suggested to be used as an initial screening tool for aromatic hydrocarbon content in bio-oil samples. In this procedure, a water-insoluble, dichloromethane-soluble fraction containing low molecular mass lignin compounds and aromatic hydrocarbons is first isolated using solvent fractionation. When the dichloromethane is allowed to evaporate at room temperature, the consequent mass loss of the pyrolysis products exhibits a linear correlation with the content of aromatic hydrocarbons in bio-oil, as quantified by gas chromatographic methods.