Barriers to racemization of the o-mono and O'-disubstituted N-phenyl ketenimines la–j were measured by dynamic NMR spectroscopy. The structures of the ketenimines 1k, 1were established by X-ray diffraction analysis. According to the experimental results and ab initio calculations (STO-3G basis set) N-aryl ketenimines and N-aryl imines in general invert their configurations by a coupled mechanism: a