The effect of nitrate and chloride ion on the rate of oxidation of ferrocene (Fc) and 1,1'-dimethylferrocene (DmFc) by Cu(II) has been studied in 95% and 80% acetonitrile/water solutions. The complex formation constants for Cu(II) with the same anions in the same media have been determined by spectrophotometry. Nitrate ion mildly inhibits the reaction, while chloride ion substantially increases the rate. The results have been analyzed in terms of the Marcus theory, and it is concluded that complexation increases the rate of self-exchange between the Cu-II(X)(n) and Cu-I(X)(n) species. In the case of nitrate, the latter effect is compensated for by a less favorable overall equilibrium constant, which results in mild inhibition.