Abstract Three different flash vacuum pyrolysis (FVP) set-ups were employed for the study of the thermal behavior of alternant and nonalternant polyclic aromatic hydrocarbons (PAH). The 1,2-switch of carbon atoms in the thermal automerizations of PAH, previously observed in benzene, pyrene and benz[ a ]anthracene, has been additionally observed for [1- 13 C]anthracene, [6- 13 C]benzo[ c ]phenanthrene and [1- 13 C]- and [4- 13 C]phenanthrenes at temperatures between 900 and 1100°C. Bay region PAH (chrysene, benz[ a ]anthracene, phenanthrene, picene, perylene, benzo[ ghi ]perylene, benzo[ e ]pyrene, and benz[ a,c ]-anthracene) were discovered to undergo an intriguing transformation when heated at 1100°C; the transformation is believed to be triggered by cyclodehydrogenation across the bay region. Thermal formation of five-membered rings was used to synthesize a series of new nonalternant hydrocarbons. When the thermal closure of a five-membered ring is energetically unfavorable, migration of an ethynyl group from one benzene ring to another along the edge of a PAH (e.g. anthracene) can take place.