Chemical and physical effects of postcondensation filtration on pine and cottonwood pyrolysis oil were investigated in conjunction with a 3-week accelerated (80 degrees C) aging study. Pine and cottonwood pyrolysis oil (produced from clear wood, whole tree, or bark biomass) underwent serial filtration to remove fine particulates. Neat (as produced) and filtered pyrolysis oil samples were then aged at 80 degrees C for time intervals ranging from 2,4 to 504 h. Physical characterization included pH, water content, and viscosity measurements. Attenuated total reflectance Fourier transform infrared (ATR-FTIR) spectroscopy, gel permeation chromatography (GPC), and coupled gas chromatography-mass spectroscopy (GC/MS) were used to identify and monitor chemical composition. Filtration alone (without aging) was found to decrease water content and subsequently increase viscosity, an effect most likely due to filter wetting. Water content and average molecular weight increased with aging time in all unfiltered and filtered pyrolysis oil samples, regardless of the starting biomass composition. Upon aging, viscosity of the filtered samples increased in all samples excluding pine whole tree fractionated (PWTF) which decreased in viscosity during aging. Aging-related changes in the viscosity correlated directly with water content and molecular weight changes for all pyrolysis oils studied, with the exception of PWTF. A major finding is that postcondensation filtration did not prevent or retard property changes that typify "aging" in pyrolysis oil, namely, increases in viscosity, water content, and molecular weight. In an attempt to better understand these physical properties in relationship to the woody biomass (sub)types used as feedstocks, the chemical composition of the pyrolysis oils was monitored using FTIR spectroscopy and GC/MS both prior to and during the aging study. Pyrolysis oils produced from clear wood presented more dominant chemical signatures from ketone, aldehyde, and/or carboxylic acid species versus pyrolysis oils produced from bark-containing biomass, demonstrating pyrolysis oil chemical composition differences due to the tree species and tree parts included in the feedstock. During aging, all pyrolysis oil samples, regardless of feedstock, exhibited decreased alcohol and/or ether content and increased concentrations of aromatics, ketones, and/or carboxylic acids. Chemical similarities were identified in the aging reactions occurring in these oils even when produced from different tree species and woody biomass compositions (i.e., clear wood, bark, whole tree). Potential reactions are discussed that match the observed increases in water content and molecular weight during accelerated aging, including air oxidation of alcohols and aldehydes to form acids and polycondensation.