The objective of this study was ro provide background information on biomass pyrolysis oils (bio-oils) regarding their use as a liquid fuel for gas turbine applications. The bio-oil was obtained by vacuum pyrolysis of softwood bark residues, Alkali metal content, viscosity, solid content, heating value, surface tension, moisture content and density of the bio-oil were investigated. The effect of the addition of methanol and/or a pyrolytic aqueous phase on the physicochemical properties of the bio-oils was also investigated. The pyrolytic aqueous phase is the sum of the water contained in form of moisture in the feedstock plus the water formed during biomass pyrolysis reactions. The results indicated that the bio-oil sample investigated is a valuable gas turbine fuel: it has a relatively low Na+K+Ca content (21ppm), a low viscosity (5.3 cSt@90 degreesC), a high net heating value (32 MJ/kg, as-received basis) and a low solid content (0.34 wt%). The addition of methanol to the oil was beneficial, It was also found that the pyrolytic aqueous phase addition had no significant effect on the viscosity, but that its "flowability" effect was beneficial for other properties. A concentration of 10-15% of the aqueous phase in the bio-oil seemed to be optimal. The second phase of this study investigated the storage and thermal stability of bio-oils and their mixtures. This was carried out using a method performed in our laboratory. The results are presented in Part II of this study.