In studies on the solution-phase photochemistry at 185 nm of cyclic monoolefins1-3 and cyclic diolefins (both conjugated4 and unconjugated5), it was found that the lifetimes of the electronically excited state that was involved were too short to promote bimolecular processes in which an electronically excited molecule was a reactant. Even intramolecular transfer of electronic energy was not observed to influence the photochemistry of a compound such as 4-vinylcyclohexene.5 In more rigid bichromophoric molecules such as 1, an internal [2 + 2] addition is a major reaction at 185 nm,5 but this may not be related to an actual transfer of electronic energy between the double bonds. In this communication, results on the photochemistry of bicyclo[4.1,0]hept-3-ene (2, hereafter BCH) at 185 nm are reported. These demonstrate that the incident photon is most probably absorbed by the π bond while the reactions that are observed are mostly those of the cyclopropyl group. © 1979, American Chemical Society. All rights reserved.