The energy profile for the carbon-to-carbon proton transfer from acetaldehyde to acetaldehyde enolate has been studied by semiempirical and ab initio methods. The ion-dipole complex is about 13 kcal/mol(-1) below the separated reactants. It has what is best described as a head-to-tail (enolate oxygen to alpha-carbon of acetaldehyde) orientation, though the relative positions of the two components of the complex are strongly dependent on level. The barrier height (transition structure complex) is low by AM1 (5.7 kcal mol(-1)) and PM3 (1.4 kcal mol(-1)) but is substantial at all ab initio levels. The barriers at 3-21G to 6-31+G* Hartree-Fock levels run 21-25 kcal mol(-1), but Moller-Plesset corrections lower the 6-31+G* barrier considerably to 13-14 kcal mol(-1) at MP2. The barrier rises to 15 kcal mol(-1) at MP4SDTQ/6-31+G**//MP2/6-31+G*, the highest level examined. There are two almost equally energetic transition structures : trans-anti and cis-gauche.