The identity proton transfer between acetaldehyde and its enolate ion has been studied by ab initio methods. Two transition states, a "cis-gauche" and a "trans-anti", have been calculated. Both transition states are characterized by a charge imbalance in the sense that charge delocalization into the carbonyl group of the incipient product enolate ion lags behind proton transfer or charge localization on the alpha-carbon of the reactant enolate ion is ahead of proton transfer. The imbalance for the cis-gauche TS, which is a fully optimized structure, is larger than that for the trans-anti TS. This is a consequence of only partially optimizing the structure of the trans-anti TS by constraining the cu-carbon to a planar geometry; hence, the trans-anti TS represents a model for a more delocalized transition state. At the MP2/6-311+G** level, the cis-gauche TS is 10.5 kcal/mol lower in energy than the trans-anti TS. Possible reasons why the less delocalized and more imbalanced cis-gauche TS is more stable than the more delocalized trans-anti TS are discussed. The reaction paths implied by the imbalanced transition states are conveniently described by means of 6-corner, hexagonal More O’Ferrall-Jencks diagrams. Corners 1 and 4 define the reactants and products, respectively; corners 2 and 3, which are 13 kcal/mol above the reactants/products, represent a hypothetical intermediate with the geometry of the acetaldehyde with the negative charge localized on the alpha-carbon ("aldanion"); and corners 5 and 6, which are 25.4 kcal/mol above the reactants/products, are approximated by the polarized structure H+CH2=CH-O- with the geometry of the enolate ion ("enaldehyde"). The trans-anti TS also shows a structural imbalance in the sense that C-C pi-bond formation in the incipient product enolate ion is somewhat ahead of proton transfer; no such imbalance is found for the cis-gauche TS.