A qualitative method for assigning the longest wavelength band in the absorption spectra ofα- andβ-monosubstituted naphthalene derivative to polarizations in definite directions has been devised. The longest wavelength band in the absorption spectra ofα-substituted derivatives is shown to be due to polarization along the short axis of the molecule; in the spectra of theβ-substituted derivatives the longest wavelength band is due to polarization along the long axis of the molecule. The absorption spectra of four isomeric hydroxynaphthoic acids have been determined, using dioxane as solvent. The deeper colour of 2:3-hydroxynaphthoic acid in comparison with the isomeric acids is explained as due toβ:β-disubstitution in the naphthalene nucleus. The absorption spectra of 2-methoxy-1-naphthoic acid, 2-methoxy-3-naphthoic acid and 2-methoxy-6-naphthoic acid are discussed.