Comparison of the catalytic effect of Mg2+ in reduction of O-2 by 1,5-dihydroflavin anion with that in electron transfer from decamethylferrocene to O-2 has revealed that the catalysis of Mg2+ in both cases is ascribed to the 1:1 and 1:2 complexes formed between O-2(.-) and Mg2+, which result in acceleration of the rate of electron-transfer reduction of O-2 to O-2(.-) in MeCN.