A hydrogen bond formed between the superoxide anion and the ammonium ion (NH4+) accelerates electron transfer from the C-60 radical anion to oxygen significantly, whereas the tetra-n-butylammonium ion has no ability to form a hydrogen bond with the superoxidie anion, exhibiting no acceleration of the electron-transfer reduction of oxygen. The second-order rate constant of electron transfer from C-60(.-) to O-2 increases linearly with increasing concentration of NH4+. This indicates that O-2 (.-) produced in the electron transfer from C-60 to O-2 is stabilized by 1: 1 complex formation between O-2(.-) and NH4+. The 1: 1 complex formed between O-2(.) and NH4+ was detected by ESR. The binding of O-2(.-) with NH4+ results in a positive shift of the reduction potential of O-2 with increasing concentration of NH4+, leading to the acceleration of electron transfer from C-60(.-) to O-2.