The identity carbon-to-carbon proton transfer between oxygen-protonated acetaldehyde (syn and anti) and acetaldehyde enol (syn and anti) has been studied by nb initio methods al the 6-311+G**//6-311+G**, MP2/6-311+G**//6-311+G**, and MP2/6-311+G**//MP2/6-311+G** levels, Previous calculations on the proton transfer between acetaldehyde and its enolate ion have been extended to the MP2/6-311+G**//MP2/6-311+G** level, On the basis of Mulliken and natural population analysis charges, the transition states of all reactions under study show a strong imbalance in the sense that charge shift in the product enol lags behind proton transfer and charge shift in the reactant enol is ahead of proton transfer, The imbalance in the reactions of CH3CH=OH+ is larger than in the reaction of CH3CH=O, and larger for the syn than the anti configuration of CH3CH=OH+. AL the highest level of calculation, the enthalpy difference, Delta H, between the transition Slate and separated reactants is about -5 kcal/mol (anti) and -2 kcal/mol (syn) for the reactions of CH3H=OH+, which compares with Delta H approximate to 0 kcal/mol for the aldehyde reaction.