Studies of aging processes were conducted on pyrolysis oils produced from pine and cottonwood biomass (clear wood, whole tree, bark and needles/leaves). Accelerated aging at 80 °C for up to 504 h was employed to investigate the short and long-term effects of feedstock, phase separation, char particulates, and solvent addition on pyrolysis oil properties. Feedstock containing forestry residue was found to increase water content of neat pyrolysis oil and the collection method (total vs. fractionated) affects all of the properties with the largest impact on viscosity and as produced molecular weight. Post-condensation liquid filtration did not prevent aging-related water content or molecular weight increases during aging but did retard aging reactions in pine clear wood and pine bark pyrolysis oils. Methanol addition retarded the aging reactions in pine needle fractionated pyrolysis oil; at 15 wt% phase separation was prevented and molecular weight increased 11 % after 504 h of aging.