Mechanisms for Stone-Wales rearrangements (SWRs) in polycyclic unsaturated hydrocarbons containing a pentafulvalene core have been studied using density functional, coupled cluster, and multiconfigurational methods. At the BD(T)/cc-pVDZ//(U)M06-2X/cc-pVDZ level of theory, free energies of activation (at 1000 °C) range from ca. 70 kcal/mol for the model system pentafulvalene → naphthalene (1 → 2) to >110 kcal/mol for the degenerate SWR of pyracyclene (3). Systems studied that do not contain a pyracyclene subunit are predicted to have ΔG(‡) less than about 90 kcal/mol and to proceed by a carbene-type mechanism. Substrates containing a pyracyclene subunit should proceed via a cyclobutyl mechanism, and appropriate benzannelation of 3 lowers the activation free energy considerably. Computed ΔG(‡) values are consistent with experimental observations reported for known systems. SWRs of two untested substrates, cyclopent[fg]aceanthrylene (18) and dicyclopenta[fg,op]tetracene (21), are predicted to have ΔG(‡) < 95 kcal/mol and thus to be accessible via flash vacuum pyrolysis.