A systematic study on the effect of various substituents at the 7- and/or 8-position on the redox properties of isoalloxazines (flavins) is reported. The redox properties of these flavin derivatives were studied by cyclic voltammetry in 100 mM, pH 7.4 HEPES and 200 mM, pH 10 berate buffers. The magnitude and direction of the effect was dependent on the nature and location of the substituent. The redox potentials of the substituted flavins were correlated with the Hammett sigma value of the substituents.