We theoretically investigate the mechanism of tautomerism in the gas-phase acetylacetone molecule. The minimum energy path between the enolone and diketo forms has been computed using the Nudged-Elastic Band (NEB) method within the density-functional theory (DFT) using the projector augmented-wave method and generalized gradient approximation in Perdew-Wang (PW91) parametrization. The lowest transition state as well as several intermediate geometries between the two stable tautomers have been identified. The outer-valence ionization spectrum for all determined geometries has been computed using the third-order non-Dyson algebraic diagram-matic construction technique. Furthermore, the oxygen core-shell ionization spectrum for these geometries has been obtained with DFT and the Becke three-parameter Lee-Yang-Parr (B3LYP)