SYNOPSISIn order to get a closer understanding of the conformational features displayed by polyamides obtained from different p-amino and p-imino acids, a systematic investigation was carried out with three sets of model Bdiamides. III(Me) assumed a folded structure. In this latter case, however, it was shown that the folding is maidy governed by a N-H . . . n interaction between the N-H group and the R orbital of the tertiary amide function.On the other hand, the N-methylation of the acetylamino group in models II(R) produces preferentially an eight-membered ring chelate which is stabilized by an intramolecular hydrogen bonding involving the N-H of the C-terminal amide function and the highly nucleophilic C=O in the N-methylated acetylamino group.Each of the evidenced conformers is described by a set of three rotational angles and is experimentally characterized by its specific spectroscopic features.