Irradiation of dissociating cyclopentadienone-dimers (2 b, c, e) results in formation of two types of cage-products (5 and 8) in very low quantum yields and good, but wavelength and phase dependent chemical yields. Both processes are intramolecular, for 2b this was shown using isotopic labeling and massspectrometric techniques. Sensitization and quenching experiments indicate that triplet states are