In this paper, we discuss the stability, structure, and vibrational assignment of possible keto forms (Ket1 and Ket2) of acetylacetone, AA, in the gas phase and solutions. The geometry optimization and harmonic and anharmonic vibrational frequencies are calculated at the B3LYP/6-311++G** level. In addition, we also optimize the molecular structure at the MP2/6-311++G(p,d) level. The investigation in solutions is carried out by means of the PCM-SCRF method. We study the relative stability of Ket1 and Ket2 in different media using B2PLYP/6-31+G(d,p) and CBS-QB3 levels. By comparing the IR spectra of AA in a polar solvent, CH3CN, and a nonpolar solvent, CCl4, and considering the theoretical results, the vibrational band frequencies of both keto and enol tautomers are distinguished. Our calculations and vibrational spectra confirm the coexisting of two keto forms in the solutions, designated as Ket1 and Ket2. According to both theoretical and experimental results, Ket1 and Ket2 are predominant in the nonpolar and polar solutions, respectively. In addition, we also do a normal coordinate analysis by using the normal mode eigenvectors obtained at the B3LYP/6-311++ G(d,p) level. The observed vibrational wavenumbers, IR and Raman relative intensities, and Raman depolarization ratios of AA and its deuterated analogous agree satisfactorily with the calculated results obtained at the B3LYP/6-311++G(d,p) method.