This study evaluates the effects of storage conditions on physical and chemical properties of biomass fast pyrolysis oils exposed to elevated temperatures over extended periods of time. It was performed on oak pyrolysis oil generated in the NREL vortex reactor. Oil samples were stored at three temperatures: 37, 60 and 90 degrees C in glass vessels. Properties of the oils were measured after hours of storage at 90 degrees C, and after days or weeks at lower temperatures. Chemical changes in the oils were measured using GPC (molecular weight distribution) and FTIR spectroscopy. The oil remained a single phase throughout the studied conditions. its pH was not affected by storage. The water content, viscosity and molecular weight of the oil increased with the time and temperature of storage. First-order reaction kinetics were successfully used to predict changes in molecular weight of the stored oil. FTIR provided evidence that etherification or esterification are mechanisms for condensation of the oil during storage.