A blue Fe3+-reduced diphosphopyridine nucleotide complex is formed very rapidly in acid solutions and disappears with first-order kinetics. The complex has two absorption bands at 540 and 375 mμ with extinction coefficients of 900 and about 6000 M-1 cm-1, respectively. Stoichiometric titrations and model building studies indicate that bidentate complexing of iron with one, two, and three reduced diphosphopyridine nucleotide molecules is possible. The binding is presumably through the reduced nicotinamide ring nitrogen and N7 of the adenine. The disappearance of the complex measured at 540 mμ is accompanied by oxidation of reduced diphosphopyridine nucleotide and the reduction of the metal; all these reactions proceed with the same rate constant. The stoichiometry of this redox reaction, after correction for the acid modification of reduced diphosphopyridine nucleotide, is such that reduced diphosphopyridine nucleotide appears as a one-electron donor. Under these conditions oxidized diphosphopyridine nucleotide appears in an equivalent amount to ferrous ion produced. When ferric iron is in high excess or in the presence of flavin mononucleotide, reduced diphosphopyridine nucleotide serves as a two-electron donor. It is concluded that an intramolecular electron transfer occurs from ligand to metal, leading to the formation of an unstable intermediate. This intermediate may undergo disproportionation, or may reduce either ferric ions or flavin mononucleotide. The flavin mononucleotide one-electron reduction product may then transfer this electron to Fe3+. The possibility that this mechanism operates in enzymic oxidations of reduced diphosphopyridine nucleotide is discussed.