Molecular structures of acetyltetralone (AT) in the enol form were investigated based on the photochemical features in comparison with that of benzoylacetone. Laser flash photolysis of AT in fluid media showed the formation of the triplet state which was characterized by time-resolved electron paramagnetic resonance measurements. From these results, the molecular structure of AT in the enol form was concluded to be endocyclic with the C-C double bond in the tetralone moiety. (C) 2012 Elsevier B.V. All rights reserved.