We report the effects of the substituent and functional group on the tautomerizations of H3CC(X)=Y and H2C(X)CH=Y (Y = SiH2, PH, S; X = H, CH3, NH2, OH, F) at the G2 level of theory. In the Y = SiH2 series, the enol forms (H2C=CX-SiH3) are thermodynamically more stable than their counterparts, by ca. 20-30 kcal/mol, regardless of their site of substitution. In the Y = PH and S cases, the keto forms (H3CC(X)=PH, H3CC(X)=S) are thermodynamically more stable than their counterparts by ca. 2-12 kcal/mol for the I-substituted series, while for the 2-substituted series the enolization processes are either endothermic or exothermic, with their relative energies near 0. Combining these results with those from the previous study, the enolization is endothermic when the non-hydrogen atom in Y is more electronegative than a carbon atom and is exothermic if the former is more electropositive than the latter. Substituents with pi-donating ability in the 1-substituted series increase the relative energy when Y = PH and S, but they decrease it when Y = SiH2. On the other hand, in the 2-substituted series, the relative energies decrease in the sequence H > CH3 > F > OH > NH2, which is the opposite order of the increasing T-donating abilities of these substituents. The barriers to enolization in this study range from 36 to 61 kcal/mol at the G2 level of theory. The effects of the functional groups and substituents on the energy barriers are explained by interpreting Hammond's postulate in terms of the position of the transition structure along the reaction coordinate, n(T), defined by Agmon, Pearson's principle of hard and soft acids and bases, and Hammett's electron push/pull effect. By Mulliken population analysis, the electronic charge of the migrating hydrogen atom becomes gradually negative, which indicates the tautomeric interconversion is as an Hdelta- transfer (or a hydride transfer). This is in contrast with that of the analogous process that occurs for compounds of the second period (Y = O, NH, and CH2).