Simple substituted derivatives of o-xylylene 1 which are stable and characterizable in fluid solution at room temperature in the absence of oxygen are generated by the irradiation of the polycyclic benzocyclobutenes 2b, 3b, and 12 in 3-methylpentane glass (77 K). The photochemical ring opening requires rigid media and may involve a successive absorption of two photons by the starting materials. The similarity of the UV and emission spectra of 5b, in which the chromophore is restricted to coplanarity, with those of the parent o-xylylene 1 suggests that 1 is close to planar. The regular vibrational progression of ca. 1500 cm-1 seen in the absorption spectrum of 5b indicates a general increase in the lengths of the C=C bonds in the excited state. While 5b shows no tendency to thermally reclose upon standing at room temperature, irradiation at 350 nm either in rigid media or in fluid solution leads to a rapid nonstereospecific reclosure to regenerate the benzocyclobutene derivatives 2b and 3b. © 1976, American Chemical Society. All rights reserved.