AbstractMNDO and MINDO/3 calculations were used to study the photochemical formation, thermal rearrangements, and dissociation of bicyclo[2.1.0]pentenone. The “forbidden” thermal conversion to cyclopentadienone requires little activation, which accounts for the low kinetic stability of bicyclo[2.1.0]pentenone. The theoretical results seem to be compatible with the available experimental evidence for the tri‐tert‐butyl‐substituted systems and suggest an explanation for observed differences in r