Résumé : The frequencies of the 1La transitions of polycyclic aromatic aldehydes are related linearly to the energy gap between the highest bonding and the lowest anti‐bonding orbitals in these molecules calculated by an iterative Hückel‐L. C. A. O.‐method. Solutions of the same derivatives and tetracyanoethylene in chloroform show a new absorption band which may be attributed to a charge‐transfer complex. A linear relation is found between the energies of these bands and the highest occupied molecular orbital of the aldehydes. Copyright © 1970 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim