There have been many studies on the pyrolysis of biomass model compounds but most have not been performed under the high heating rate, short residence time conditions currently used in the thermochemical conversion of biomass. In this investigation, the pyrolysis of plant steroids is investigated by Β―ash vacuum pyrolysis (FVP) and Β―ow pyrolysis at residence times of 0.1Β±2.0 s and temperatures of 550Β±8008C to determine if the native cyclic ring structure in the steroid leads to the formation of polycyclic aromatic hydrocarbons (PAHs) or whether PAHs are formed by pyrosynthesis. FVP of stigmasterol, stigmasterol acetate, b-sitosterol, and stigmasta-3,5-diene at 7008C showed that PAHs, such as acenaphthylene, phenanthrene, anthracene, pyrene, chyrsene, benz[a]anthracene, and their monomethylated derivatives were formed in the absence of bimolecular reactions, and the yield of PAHs was dependent of the structure of the steroid. Similar products were found in the Β―ow pyrolysis of stigmasterol, and the yields of PAHs were found to increase with increasing temperature and residence time. Thermochemical kinetic estimates were used to postulate pathways for the formation of the primary pyrolysis products.