Pyruvic acid and its isomers, including the enol tautomers and enantiomeric lactone structures, have been investigated at the B3LYP/6-311 + + G(3df,3pd) level, and it is found that a keto form with trans C_methylC_ketoC_acidO_hydroxyl and cis C_ketoC_acidOH, and with one methyl hydrogen in a synperiplanar position with respect to the keto oxygen, is the most stable. This agrees with previous theoretical and experimental determinations. However, no minimum corresponding to protonated pyruvate could be located, although previous semiempirical calculations had found such structures. Decarboxylation by different possible routes was then studied. It was found that the direct formation of acetaldehyde, the most stable of the resulting C₂H₄O isomers, via a four-center-like transition state is the most feasible, although there is a high activation barrier of 70 kcal mol⁻¹. In contrast to semiempirical calculations, it is found that no hydroxyethylidene–carbon dioxide complex exists as a product, and no transition state leading to the dissociation to hydroxethylidene could be located.