The electroreduction of O-2 in two aprotic solvents (nitrobenzene and acetonitrile) was examined in the absence and in the presence of the very strong Lewis acid, triphenylmethyl cation, phi(3)C(+) (phi = phenyl). The addition of phi(3)C(+) causes the electroreduction of O-2 to change from the one-electron reduction to O-2(-.) into the two-electron reduction to phi(3)CO(2)C phi(3) at a potential that is 1 V more positive than that where O-2 is reduced to O-2(-.). This large positive shift in potential is used to estimate a value of the equilibrium constant for the association between O-2(2-) and two phi(3)C(+) cations (K = 3 x 10(26) M-2). The mechanism of the reduction of O-2 to phi(3)CO(2)C phi(3) can be regarded as an inner-sphere electron-transfer reaction between phi(3)C(.) and O-2 or, equivalently, as a radical addition to O-2.