UV- and IR-induced photoisomerizations of acetylacetone trapped in a nitrogen matrix at 10 K have been carried out using a tunable (UV and IR) laser, or a mercury lamp, and have been studied by U-V and FTIR spectrometries. After deposition the main form of acetylacetone is the chelated form (enol). Upon UV irradiation the intramolecular H-bond is broken, leading to non-chelated forms. These forms have been then irradiated, by a resonant II --> II* UV irradiation, or by resonant upsilon(OH) IR irradiations. Interconversions between the non-chelated forms have been observed, providing a great amount of data for identifying acetylacetone isomers and thus for exploring intramolecular reactivity paths. (C) 2003 Elsevier Science B.V. All rights reserved.