Measurements have been made of the absorption spectra and fluorescence of riboflavin in aqueous solution in the presence of varying concentrations of phenol and phenol derivatives. All measurements were carried out at pH 4.3 and 20° C. The absorption spectra of riboflavin was shifted to longer wave-lengths by phenol and its derivatives. Extinctions in the position of the normal absorption maxima and the extinction relationship E375/E450 decreased while extinction in the region between 465 and 520 mμ increased. This effect is not inhibited by a nuclear amino group, and a nuclear carboxyl group heightens the effect. Changes in spectra and fluorescence are believed due to complex formation between the phenol and the isoalloxine ring of riboflavin. The reaction is bimolecular. The dissociation constant of the riboflavin-phenol complex is 0.21 mol/l. at 20° C. The extinction of fluorescence of riboflavin by phenol is due not only to complex formation, but also to double collision.