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 O2 increases linearly with increasing concentration of NH4+. This indicates that O 2- produced in the electron transfer from C60 to O2 is stabilized by 1:1 complex formation between O 2- and NH4+. The 1:1 complex formed between O2- and NH4+ was detected by ESR. The binding of O2- with NH4+ results in a positive shift of the reduction potential of O2 with increasing concentration of NH4+, leading to the acceleration of electron transfer from C60- to O 2.