It was found that FAD or FMN formed complexes equimolecularly with metal ions such as Ca^<++>, Mg^<++>, Ba^<++>, Cd^<++>, Mn^<++>, Co^<++> and Ni^<++>. The stability constants for these complexes were determined at pH 7.2 and 0.1 of ionic strength. It was shown that the stability constants for FMN complexes containing transition metals such as Mn^<++>, Co^<++>, Ni^<++> were greater than those for the other metal complexes. However, these relations were not observed in the stability constants for FAD complexes.