High-level ab initio calculations were carried out in both gas phase and solvent (epsilon = 35.9) to estabilish that the amination of ketenimine proceeds via amine addition across the C=N bond rather than the C=C bond, followed by tautomerization to form amidine product. The HOMO of ketenimine is perpendicular to its molecular plane with the largest coefficient on C(beta), while the LUMO is in its molecular plane with the largest coefficient on C(alpha). Amination of ketenimine involves in-plane attack of amine nucleophile on C(alpha) (LUMO) of ketenimine. The labile vinylidenediamine intermediate trans-11 for the reaction of ketenimine 10 with n-butylamine was directly observed by means of low-temperature proton NMR spectra. The evidence confirms that the amination reaction is stepwise and proceeds via n-butylamine addition across the C=N bond of ketenimine 10 rather than the C=C bond, followed by a slower tautomerization of vinylidenediamine trans-11 to amidine 12. Even though the second step is much slower, the first step involving amine addition across the C=N bond is kinetic control. Surprisingly, in the reaction of 10 with n-BuNH(2), attack of n-BuNH(2) syn to the phenyl group on C(beta) of 10 is preferred, even though this produces a less stable product (trans-11); attack of n-BuNH(2) anti to phenyl group on C(beta) of 10 is lacking and results in serious nonbonding interactions between the two phenyls of the ketenimine, as they are pushed together in this transition state.