Résumé : The addition of small amounts (10-2 M) of proton donors to hexane solutions of azaaromatics induces the following changes in the U.V. spectra:(a) The disappearance of the n-π* band.(b) A hyperchromic effect on the π-π* transitions. These findings allow the non-ambiguous assignment of the n-π* transitions in pyridine (270 mμ), quinoline (318 mμ), isoquinoline (312 mμ), acridine (360 mμ) and 1-azafluoranthene (416 mμ). The agreement with L.C.A.O. calculations is excellent in the case of the azaderivatives of alternant hydrocarbons; the intensities may be interpreted qualitatively. The influence of acetic acid on the one hand, and of trichloro- and trifluoroacetic acids on the other hand, on the spectrum of isoquinoline, suggests the existence of two different types of hydrogen bonds between the base and the free acid. © 1962 Pergamon Press Ltd.