Addition of alcohols was found to improve the homogeneity, decrease the viscosity and density, lower the flash point, and increase the heating value of pyrolysis liquids. Alcohol addition also lowered the viscosity and molecular mass increase during the aging of pyrolysis liquids. The reduction in the viscosity change was primarily due to a stabilizing effect of alcohols on the water-insoluble high molecular mass lignin-derived fraction. Other effects include the formation of acetals in reactions of alcohols with aldehydes, ketones, and anhydrosugars. Low (less than or equal to5 wt %) alcohol additions prevented aging reactions by a few months, while the higher (greater than or equal to10 wt %) ones retarded them by almost a year. Methanol was the most effective alcohol of those tested (methanol, ethanol, isopropanol). By improving solubility, the alcohols also enhanced the separation of the extractive-rich top layer in the pyrolysis of forestry residue by decreasing its volume and increasing the concentration of extractives and solids in the top layer.