Reaction conditions are presented that, for the first time, allow the generation and dimerization of N-alkylimidoylketenes, e.g., 1d, while avoiding the intramolecular rearrangements observed under conventional conditions. The dimer of 1d (22a) is the result of [4 + 2] cycloaddition across the C=C bond of one ketene. In contrast, the N-H imidoylketene 1c dimerizes across the C=O bond to form 24b. Furthermore, N-methylbenzoimidoylketene (5b), in equilibrium with the more stable benzoazetidinone 14b, gives the formal [4 + 4] dimer 8b. B3LYP/6-31G(d) transition structure calculations on these three modes of dimerization reproduce and offer explanations for these divergent regiochemistries. Both [4 + 2] dimerizations have planar, pseudopericyclic transition structures (25a and 29b). Five transition structures were found for the formation of 8b. A unique pseudopericyclic dimerization of 5b with an orthogonal [4 + 4] geometry (31) has a barrier of only 0.7 kcal/mol. However, the overall lowest energy pathway involves concerted addition of 5b across a σ bond in 14b via 35.