Geometries of the 12 possible cycloaddition products between ketenimine and acrolein as well as of the respective transition states have been obtained by ab initio calculations at the MP2/6-31G* level. Energies were obtained up to the MP4(SDTQ)/6-31+G* level. On the basis of product stability arguments, addition of the ketenimine C=C double bond is more favorable than reaction of the C=N double bond. Similarly, acrolein also preferentially should react via its C=C rather than C=O bond. In kinetically controlled reactions, however, participation of the ketenimine C=N bond in [4+2] cycloadditions across the oxa-1,3-dienic system is comparable to (8) dr far more feasible than (7) reaction of the C=C bond.