Benzoazetinone was photochemically generated by UV irradiation of isatin isolated in low-temperature Ar matrixes. Upon UV (lambda = 278 nm) excitation of isatin, monomers of the compound underwent decarbonylation and the remaining part of the molecule adopted the benzoazetinone structure or the structure of its open-ring isomer alpha-iminoketene. The same products (benzoazetinone and alpha-iminoketene) were generated by UV (lambda = 278 nm) induced decarboxylation of matrix-isolated monomers of isatoic anhydride. Photoproduced alpha-iminoketene appeared in the low-temperature matrixes as a mixture of syn and anti isomers. Photoproducts generated upon lambda = 278 nm irradiation of matrix-isolated isatin were subsequently exposed to lambda = 532 nm light. That irradiation resulted in the shift of the alpha-iminoketene-benzoazetinone population ratio in favor of the latter closed-ring structure. The next irradiation at 305 nm caused the shift of the alpha-iminoketene-benzoazetinone population ratio in the opposite direction, that is, in favor of the open-ring isomer. Neither benzoazetinone nor its alpha-iminoketene open-ring isomer was generated upon UV (lambda = 278 nm) irradiation of phthalimide isolated in Ar matrixes. Instead, the UV-excited monomers of this compound underwent such phototransformations as oxo -> hydroxy phototautomerism or degradation of the five-membered ring with release of HNCO and CO. The efficiency of these photoconversions was low.