Infrared spectra of some flavonoids and quercetin and morin sulfonic derivatives were recorded. The spectral data of sodium salts of quercetin and morin sulfonic acids were interpreted as indicating the near position valency band of the carbonyl group in relation to its position in quercetin and morin. The distinct shift of this band in quercetin-5'-sulfonic acid (QSA) may probably be a result of the occurrence of an intramolecular proton transfer from the sulfonic -SO3H to the oxygen of -C=O group.