A method was developed to simulate the rapid heating bio-oil fractions will undergo if upgraded using conventional petroleum refining processes. Bio-oil fractions were produced via fluidized bed fast pyrolysis of southern yellow pine sawmill residue. Thermal processing of the bio-oil fractions was evaluated at three temperatures (100, 200, and 300 degrees C) and two heating times (60 and 120 s). Thermal stability was defined as the increase in average relative molecular weight (RMVV) of bio-oil samples after thermal treatment. The effect of moisture content and total acid number (TAN) on thermal stability was also investigated. Changes in chemical structures were observed via Fourier transform infrared spectroscopy (FTIR). Bio-oil fractions exhibited considerable instability at temperatures above 100 degrees C with substantial increases in average RMW for both 60 and 120 s heating times. The initial concentration of acids, as measured by TAN and ion chromatography (IC), was found to be the strongest predictor of thermal instability.