B3LYP/6-31G(d,p) electronic structure calculations are employed to elucidate the reaction mechanisms for the conversion of the alternant C18H12 polycyclic aromatic hydrocarbon benzo[c]phenanthrene into the nonalternant C18H10 PAHs cyclopenta[cd]pyrene and benzo[ghi]fluoranthene. Isomerization reactions such as 5/6-ring switching and hydrogen atom scrambling are analyzed. Bay region chemistry, involving the rupture of one benzene ring followed by the formation of a new five-membered ring, is also studied, together with the mechanism for the formation of an aryne. The rearrangement of the latter yields annelated cyclopenta-dienylidenecarbene, which is then trapped intramolecularly.