Photoisomerization mechanism of 4-amino-3-penten-2-one, a chelated keto-enamine isomer, has been studied by low-temperature matrix-isolation infrared (IR) spectroscopy. Conformations of the reactant and two photoproducts were determined by comparison of the observed IR spectra with the calculated spectral patterns obtained by density-functional-theory (DFT) calculations. It is concluded that two isomers are produced upon UV irradiation: a non-chelated keto-enamine isomer by internal rotation around the CH=C bond with breaking the intramolecular hydrogen bond, and an imino-enol isomer by hydrogen-atom migration from the amino group to the carbonyl group. (c) 2006 Elsevier B.V. All rights reserved.