It is well known that the pretreatment of biomass prior to fast pyrolysis process can alter the structure and chemical composition of biomass feedstocks leading to a change in the mechanism of biomass thermal decomposition. As a result, bio-oils with quite different chemical compositions can be obtained, and these bio-oils may be used for varied applications. We determined the effect of six pretreatment processes (dilute phosphoric acid, dilute sulfuric acid, sodium hydroxide, calcium hydroxide, ammonium hydroxide and alkaline hydrogen peroxide) on the chemical and physical characteristics of bio-oils. Bio-oils were produced from pine wood feed stocks in an auger reactor at 450°C temperature. Proximate, ultimate and heating value analyses were performed for both biomass feed stock and the produced bio-oils. The chemical composition the bio-oils were also determined by gas chromatography-mass spectrometry (GC-MS), gel-permeation chromatography (GPC) and Fourier Transform Infrared (FT-IR) techniques. Results showed that the chemical species produced were highly influenced by the biomass pretreatments applied. These chemical changes are compared and discussed in detail in this paper.