Pyrolysis and photolysis of derivatives of 2,4-dibenzylidenebicyclo[3.1.0]hexan-3-one cause isomerization at the cyclopropyl ring, consistent with formation of a pentamethylenepropane (PMP)-type intermediate or transition state. Kinetic studies of the isomerization gave E(a) = 31 kcal/mol and log(A) = 11.2, consistent with reasonable thermodynamic estimates for the formation of a PMP-type species. No evidence was found under rigid cryogenic conditions for a triplet PMP in photolysis of the bicyclo[3.1.0]hexan-3-ones. Computational studies indicate that bicyclic PMP derivatives 1,2,3-trimethylenecyclohexane-4,6-diyl and 2,6-dimethylenecyclohexanone-3,5-diyl have B-3(2) ground states, by <2 kcal/mol relative to their respective diradical (1)A(1) states. This close spacing of the singlet and triplet energy manifolds is probably in part responsible for the ineffectiveness of generation of the triplet PMPs from singlet bicyclo[3.1.0]hexan-3-one precursors.