UV-induced photoisomerization of acetylacetone in low-temperature argon matrixes has been studied by Fourier transform infrared spectroscopy. Identifications of the species produced by UV irradiation (A > 280 nm) were carried out with the aid of the density functional theory (DFT) calculation, in which the 6-31G* basis set was used to optimize the geometrical structures. By comparison of the observed infrared bands with the calculated spectral patterns, it was found that cis-trans isomerization around the C-C, C=C, and C-O bonds occurs to produce less-stable enol isomers, 2-hydroxy-2-penten-4-one. Shorter-wave length irradiation (lambda < 280 nm) induced hydrogen-atom migration of the enol isomers to produce a keto isomer, 2,4-pentanedione.