All reasonable tautomers and rotamers of tenuazonic acid, which is considered to be of the highest toxicity amongst the Alternaria mycotoxins, were investigated by OFT calculations at different levels of theory in gas phase and in solution to obtain optimized geometries for further examinations. Calculated NMR spectra of tautomeric structures are being presented and compared to experimental data to finally achieve a synergetic computational and spectroscopic approach for structure elucidation of 3-acetyltetramic acids, affording the predominant tautomer of tenuazonic acid in aqueous solution. Furthermore we were able to simulate the less hindered rotation of the exocyclic acetyl group, which occurs after dissociation of tenuazonic acid in protic solvents. (C) 2013 Elsevier B.V. All rights reserved.