[en] Flavin-adenine dinucleotide (FAD) and flavin mononucleotide (FMN) were irradiated with ⁶⁰Co γ-ray and G-values for their decomposition under aerated, argon and nitrous oxide saturated conditions at pH 6.5 were determined by spectrophotometric, fluorescence and HPLC measurements. Among the water-borne radicals the hydroxyl radical (OH) was found to be responsible for causing permanent changes in FAD and FMN. The striking similarity between the difference spectra of irradiated FAD and FMN in N₂O saturated conditions shows that the OH radicals attack at the isoalloxazine moiety. A plausible mechanism for the reaction of the OH radicals with the isoalloxazine moiety has been discussed. The rate constant for the reaction of isoalloxazine moiety of FAD with OH radicals was determined to be 4.7 x 10⁹ dm³ mol^-1s^-1 by competition kinetic studies using glucose as the reference solute. From the kinetic consideration and G-values it is evident that ca. 50% of the OH radicals attack at the AMP (adenosine monophosphate) portion of FAD apart from isoalloxazine moiety. The attack at the AMP portion of FAD is supported by the observed phosphate release in the irradiated FAD solution. Effect of Fe³⁺ ion on the radiolysis of FAD and FMN was also studied. (author)