Résumé : The infrared absorption of aza polycyclics (1.000 cm−1 to 650 cm−1) shows characteristic bands, assigned to the C–H out‐of‐plane vibrations, which are classified according to the number of groups of neighbouring hydrogens. The correlation with similar assignements used in the field of homocyclic aromatic hydrocarbons is shown. Monodeuterated polycyclics show the same pattern in the considered region of the spectrum. Copyright © 1961 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim