Ab initio molecular orbital calculations have been performed on the alpha-substituted acetaldehydes XH2CCH=O (X=H, BH2, CH3, NH2, OH, F, CN, NC, and Cl) to investigate the substituent effects on the keto-enol tautomerisms. Structures for all stationary point (ketones, enols, and transition states) were optimized and characterized at the MP2(full)/6-31G* and MP2(full)/6-31G** levels of theory. intrinsic reaction coordinates (IRC) calculations were performed in order to connect transition structures with the appropriate tautomeric pairs. Results from various levels of calculations all show that the keto forms are thermodynamically more stable than the enol forms. At the G2 level, the former tautomers are energetically favored over the latter forms by 2.9, 5.6, 7.8, 9.6, 9.7, 10.2, 10.4, 11.8, and 12.1 kcal/mol for X = BH2, CN, NC, NH2, CH3, OH, Cl, H, and F, respectively. All substituents except F stabilize the enol form relative to its keto counterpart as shown by the reduction of the energy gaps between the former and the latter forms. At the same G2 level, the respective activation energies of enolizations relative to the keto form were found to be 42.2, 58.0, 60.8, 61.8, 62.4, 63.3, 63.8, 64.5, and 65.3 kcal/mol for X = BH2, CN, NC, NH2, Cl, OH, CH3, H, and F. Except for X = F, the alpha-substituted aldehydes all lower the barrier to the tautomeric interconversions. The substituent effects on the energetics in this study were compared with the results obtained from our previous theoretical investigations on the tautomeric interconversion of CH3COX and XCH2CHNH where X = BH2, CH3, NH2, OH, F, CN, H, and Cl.