The g values of ESR spectra of superoxide-metal ion complexes (O2(*-)-Mn+, n = 1, 2, 3) are determined in acetonitrile at 143 K. The binding energies (deltaE) of metal ions with O2*- have been evaluated from deviation of the gzz values from the free spin value. The deltaE values are well correlated with the catalytic reactivities of metal ions in electron transfer from cobalt(II) tetraphenylporphyrin to O2 and p-benzoquinone, which does not occur in the absence of metal ions under otherwise the same experimental conditions. The deltaE values can thereby be used as the first quantitative measure for Lewis acidity of metal ions in relation with the catalytic reactivities in electron transfer reactions.