Ab initio molecular orbital calculations have been carried out on the alpha-substituted acetaldimines XH(2)CCH=NH (X = H, BH2, CH3, NH2, OH, F, Cl, CN, NO) to investigate the substituent effects on the imine reversible arrow enamine tautomerisms. All stationary points (imines, enamines, and transition states) were optimized at HF/6-31G(*) and MP2(full)/6-31G(*) levels of theory and subsequently characterized by the frequency calculations. Single-point calculations at the MP4(FC)/6-31G(*)//MP2(full)/6-31G(*) and MP4(FC)/6-311 ++ G(**)//MP2(full)/6-31G(*) levels were performed for all stationary points. The intrinsic reaction coordinates (IRC) were traced in order to connect the transition structures and the corresponding substituted tautomers. MP2(full)/6-31G(*) results favor the imines over the enamines by 7.89, 6.56, 5.95, 5.61, 5.57, and 5.55 kcal/mol, respectively, for X = H, F, OH, Cl, NH2, and CH3. While, for X = BH2, CN, and NO, the enamines are lower in energies than the imines by 5.67, 1.20, and 3.77 kcal/mol, respectively. Analysis of NBO calculations shows the empty 2p atomic orbital in boron and the pi(*) orbitals in CN and NO interact with the adjacent C=C double bond.