Accelerated aging was examined over a broad temperature range (40-290 degrees C) to determine the stability and different aging mechanisms for pyrolysis oil produced using fluid catalytic cracking (FCC) of solid particulate biomass material. Oil properties, including water content, high-molecular-mass (HMM) lignin content, viscosity, molecular weight, and chemical composition, were measured both before and after the accelerated aging study. Notably, at all aging times and temperatures explored, all samples remained as a single phase. Chemical composition of the pyrolysis oils was monitored using infrared and mass spectroscopies. Oils aged at temperatures above 60 degrees C showed marked increases in HMM lignin content and molecular weight with increasing aging time. Calorimetry results indicated that, at and above 200 degrees C, there were alternate aging mechanisms that occur, including sugar decomposition. A rapid increase in the water content and shift in the HMM fraction molecular weight corroborated the decomposition of sugars, which led to the formation of water and reactive monomers. Fourier transform infrared (FTIR) results showed that similar aging reactions occurred for oils aged between 40 and 80 degrees C and between 150 and 290 degrees C. Gas chromatography mass spectrometry (GC-MS) results suggested the reactions among furfural, 2-cyclopenten-1-one, 2-hydroxy-3-methyl-2-cydopenten-l-one, hydroxybenzenes with available ortho/para sites, and the active sites on HMM lignin contributed to property changes as a result of heat treatment.