Photoreaction of 7-hydroxyquinoline monomer in a low-temperature argon matrix was investigated, and electronic absorption spectra of three unstable species generated by UV-light irradiation were measured. By comparison with the calculated spectral patterns obtained by the time-dependent density-functional-theory calculation, the electronic absorption bands around 460, 360 and 260 nm appearing after UV irradiation were assigned, respectively, to the keto form produced from the enol form of 7-hydroxyquinoline by migration of the hydrogen atom of the OH group to the N atom, quinolinoxyl radical by elimination of a hydrogen atom, and 7-ketene by Wolff rearrangement following elimination of a hydrogen molecule. (C) 2010 Elsevier B.V. All rights reserved.