Ultraviolet and infrared spectroscopy have been used to study the photochemistry of 1,3,5-cycloocta-triene (I) and bicyclo[4.2.0]octa-2,4-diene (IV) suspended in inert vapor matrices at 20°K. cis,cis-1,3,5,7-Octa-tetraene, benzene, and ethylene were found to be common primary photoproducts of the two parent valence isomers. Photolysis of I also yields another primary product. This product reverts to the starting material at room temperature and has a vibrational spectrum which supports its assignment as a strained cyclic stereoisomer of I. These results are discussed in conjunction with the results of previous flash photolysis and solution photochemical mechanisms involved. © 1969, American Chemical Society. All rights reserved.