Stark-effect spectroscopy (electroabsorption) measurements were obtained for oxidized flavin adenine dinucleotide (FAD) and flavin mononucleotide (FMN) in frozen glycerol/H2O glasses and N(3)-methyl-N(10)-isobutyl-7,8-dimethyl-isoalloxazine in frozen n-butanol glasses at fields of up to 5 x 10(5) V/cm. In all three flavins, the effect of the applied electric field on the low-energy transition (S-0 --> S-1, 450 nm band) is significantly smaller than on the higher energy transition (S-0 --> S-2, 370 nm band). The Stark spectra indicate that the magnitude of the permanent dipole moment in the S-1 state, /<(mu)over bar>1/ is only modestly different from the S-0 state, /<(mu)over bar>(0)/, and that there is little change in the mean polarizability for the S-0 --> S-1 transition. The electric field effect on the S-0 --> S-2 transition, however, shows that the magnitude of the dipole moment of the S-2 state is similar to 60% larger than that of the S-1 state and that the change in the mean polarizability is much larger. Concentration studies indicate that the FAD dimer or larger FAD aggregates give a nonlinear enhancement of the electric field effect. The source of this enhancement is unknown but may have to do with the stacked isoalloxazine-adenine configuration extended over a dimer or larger cluster of FAD molecules.