Bio-oil is chemically and thermally unstable during storage and transportation. For that reason, it is necessary to evaluate the changes in the properties (chemical and physical) of bio-oil during storage to understand its chemical instability, which will further assist researchers in stabilization strategies. This paper describes the evaluation of an accelerated aging process on the physical and chemical properties of bio-oil from fast pyrolysis of ash and birch woods using two different pyrolyzers, a pilot scale (auger) and lab scale (tube furnace), respectively. The produced oils (freshly made) were aged at 80 degrees C over different periods (1, 3, and 7 days) in sealed nitrogen-purged Nalgene vessels. Fresh oil was analyzed alongside aged oils. Bio-oils were characterized by viscometer, Karl Fischer titration (H2O), pyrolysis gas chromatography/mass spectrometry (GC/MS), thermogravimetric analysis (TGA), photo-microscopy, C-13 nuclear magnetic resonance (NMR), and Fourier transform infrared spectroscopy (FTIR). The water content, viscosity, decomposition temperature (TGA) and ash content levels in bio-oil samples all increased as the aging period lengthened. GC/MS analysis showed a major reduction in GC-analyzable components. The mass of residue remaining after pyrolysis GC/MS increased, and the structures of pyrolysis products of this non-volatile residue along with NMR and FTIR data suggest the following aging processes; some of the reactive compounds undergo polymerization or reaction with other compounds, including olefins, alcohols, and aldehydes. Some possible reaction mechanisms are given. The oils remained as a single phase throughout the initial study period; however, on day 7, a clear phase separation was observed by photo-microscopy.