Ab Initio calculations (MP2/6-31G*//HF/6-31G*,including zero-point vibrational energy (ZPVE) corrections and solvent effect) predict that the reaction between ketenes and carbonyl compounds takes place via unusually synchronous [pi 2(s) + (pi 2(s) + pi 2(s))] transition structures (TS’s). The reaction between formaldehyde and activated monosubstituted ketenes such as chloroketene is predicted to be more synchronous and to take place through an exo TS, whose shape is consistent with the experimentally reported low stereoselectivity in the cycloaddition between monosubstituted ketenes and aldehydes. The catalyzed reaction is predicted to be a concerted two-stage process, in which the Lewis acid in the TS is bonded to the carbonyl oxygen of the aldehyde and adopts an exo disposition with respect to the 2-oxetanone ring in formation. The shape of this latter TS accounts for the preferential formation of cis 2-oxetanones when bulky Lewis acid catalysts are used.