Résumé : Triphenylene has been chosen as a particularly suitable ring system for the preliminary study of shielding and deshielding effects induced by different kinds of substituents, in polycyclic aromatic hydrocarbons. The results are summarized in the form of diagrams in Fig. 1. In the nine cases examined so far, it is possible to distinguish derivatives with an angular substituent (position 1) from those having a substituent in a non angular position (position 2). Data from this work will be used to solve problems in more complex ring systems. © 1964.