Abstract Polarographic studies of the aerobic oxidation of catecholamines under the influence of FMN in various concentrations, and spectrophotometric investigations of the formation of FMN–catecholamine complexes, have been carried out. By following the course of the oxidation reaction polarographically, the rate constants have been found. The oxidation of catecholamines by air was found to be disturbed through complex formation and to be facilitated by light illumination. The equilibrium constants for the formation of some catecholamine complexes with FMN have been calculated.