Bio-oil has several undesired fuel properties such as high viscosity, high acidity, high molecular weight, instability, and phase separation upon aging. Further, the high oxygen content (typically 45-50 wt%) and the water content (typically 15-30 wt%) of pyrolysis oil result in a lower energy density than conventional fuel oils. Whole bio-oil from the fast pyrolysis of birch bark is phase separated at room temperature, probably because of the high extractive content of the bark. It prevents the application of bio-oil derived from birch bark as a fuel. Therefore, the main objective of this research is to investigate the application of fractional condensation of pyrolysis vapors as a promising method to obtain bio-oils that are more resistant to aging. Bio-oil vapors were separated into three cuts: a hard solid in the first condenser, a viscous oily cut ("second condenser cut") in the second condenser/electrostatic precipitator, and a water-rich acidic liquid in the third condenser. It was found that fractional condensation is an effective method to separate water, acids and other low molecular weight reactive components from bio-oil and is a promising downstream approach to control the quality of bio-oils, making it more suitable for further upgrading and/or direct application. Performing fractional condensation and mixing the second condenser oil fraction with 25% isopropanol yields a very stable liquid fuel. (c) 2020 Elsevier Ltd. All rights reserved.