We present experimental and theoretical studies of the ring-chain tautomerism (H-atom transfer and cyclization) for 2-acetylbenzoic acid at both ground and electronically first excited states. 1 H and 13 C NMR studies in solution confirm the existence of equilibrium between the open and ring structures at the ground state, with the ring one being dominant (∼90%). Temperature-dependent 1 H NMR experiments allowed obtaining the thermodynamic and kinetic parameters at the coalescence temperature (380 K). Fluorescence measurements disclose the involvement of highly efficient nonradiative processes in agreement with the theoretical data. Electronic calculations for the ground state give additional information on the different conformers of the open tautomer. In agreement with the experiment the most stable structure is of the closed ring tautomer, and it is obtained after additional internal rotations of the -COOH and -CO(CH 3 ) fragments. Intrinsic reaction coordinate calculations indicate that the ring formation/breaking and the H-atom transfer are taking place in a concerted but not synchronous manner. At S 1 the most stable form is the open one, for which different conformers are also found. The influence of the solvent is also accounted for through a model that considers the solvent as a continuum at both the ground and excited electronic states. No major differences were observed when comparing both gas and condensed phase results, so calculations of the isolated molecule should give a picture of the reaction which is experimentally observed in solution.