Instability of pyrolysis oil during aging is a significant issue for commercialization of pyrolysis technologies on an industrial scale. Hot gas filtration of pyrolysis vapors before condensation has been shown to dramatically improve the stability of pyrolysis oils. Detailed chemical analysis of oils produced under various hot gas filter temperatures and vapor residence times shows that the chemical composition of pyrolysis oil changes significantly when hot gas filtration is employed. Compounds that are known to cause instability are eliminated and more stable compounds are formed. This information provides insight into the mechanisms responsible for the stabilization of pyrolysis oils when hot gas filtration is employed.