The initial development of additives to stabilize the viscosity of biocrude during long-term storage has produced dramatic results. The additives investigated were ethyl acetate, methyl isobutyl ketone and methanol, acetone, methanol, acetone and methanol, and ethanol. These additives represent three chemical families, which all demonstrated the ability to drastically reduce the aging rate of biocrude, as defined by the increase in viscosity with time. Accelerated aging tests were run at 90 degrees C to screen the additives. The additives not only lowered the initial viscosity at 40 degrees C by half but also reduced the aging rate of a hot gas filtered pyrolysis oil made from hybrid poplar (NREL run 175) by factors of 1-18 compared to the original pure oil. With the best additive, methanol, at a 10 wt % level in the pyrolysis oil, the modified biocrude was still a single-phase liquid and still met the ASTM No. 4 diesel fuel specification for viscosity even after 96 h exposure to 90 degrees C. Eased on the aging rate at 90 degrees C recently determined for pure biocrude without additives, the pure biocrude tested would have exceeded the allowable ASTM No. 4 viscosity after only 2.6 h. In addition, the unmodified biocrude formed a waxy precipitate that floated on top of the liquid phase after 8 h exposure to 90 degrees C. Use of methanol with previously aged oils greatly reduces the resultant viscosity, but not quite as effectively as the use of the methanol shortly after the pyrolysis oil is produced. The cost of the additive, e.g., methanol, may be offset by the heating value it adds to the pyrolysis oil, depending on the local cost of each.