The mechanism and the origin of the selectivity for the BINOL-phosphoric acid-catalyzed Strecker reaction on N-benzyl imines has been investigated by theoretical methods and compared with earlier studies of N-aryl imines. ONIOM calculations show that the reverse in selectivity which is observed experimentally is not due to differences in the steric bulk of aryl and benzyl groups, but rather because of a switch from a preference for a Z imine to an E imine in the transition state of benzaldehyde-derived imines. The calculations predict this change will not be present for imines formed from acetophenone.