Photoisomerization of hexafluoroacetylacetone was investigated by a low-temperature matrix-isolation technique with an aid of the density functional theory (DFT) calculation. A less stable isomer produced by 'LN irradiation (lambda > 260 nm) from the most stable enol was identified by its IR and UV-visible absorption spectra. This isomer was found to be non-chelated enol with cis, trans, and trans conformations around the CO-CH, CH=C, and C OH bonds, respectively, by a comparison with the spectral patterns of seven possible isomers with non-chelated enols derived from DFT simulations. (C) 2003 Elsevier Science B.V. All rights reserved.