The protonation of 2,3,4,5‐tetraphenylcyclopentadienone (tetracyclone) yields 6,11‐diphenyl‐5 H ‐benzo[ a ]fluoren‐5‐one ( 17 ) and 2,3,4,5‐tetraphenylcyclopent‐2‐en‐1‐one ( 18 ) as the major products. This contrasts with the pyrolysis of tetracyclone, which yields 5,6‐diphenyl‐11 H ‐benzo[ a ]fluoren‐11‐one ( 22 ), a structural isomer of 17 . Mechanisms are presented that rationalize these observations. The protonation of 4‐hydroxy‐2,3,4‐triphenylcyclopent‐2‐en‐1‐none, the precursor to 2,3,4‐triphenylcyclopentadienone, generates 2‐(2′‐oxo‐3′,4′,5′‐triphenylcyclopent‐3′‐enyl)‐3,4,5‐triphenylcyclopenta‐2,4‐dienone ( 30 ), which has been unequivocally characterized by X‐ray crystallography. The establishment of the conformation of molecule 30 in the solid state, which correlates with the conformation in solution, provides a rationale for the subsequent formation of the tetrahydro‐pentaphenyl‐ as ‐indacene‐3,4‐dione ( 32 ) upon further protonation.