The effects of the addition 10 −4−10 −1 M of various anions and cations to i N H 2S0 4 or I M NaOH solutions on oxygen-reduction at pre-anodized and pre-cathodized platinum electrodes were examined by measuring coulombic efficiency for formation of hydrogen peroxide (calculated on the basis of O 2+H 2O + 2 e → OOH − + OH −) and also by current-potential curves. In I M NaOH, oxygen was reduced to hydrogen peroxide with an efficiency of almost 100% at the pre-anodized electrode. However, in the presence of Ba 2+,Sr 2+, and Ca 2+ in I M NaOH, the efficiency decreased to 10–15% with increasing concentration of the ions. In I N H 2S0 4, oxygen was reduced mainly to water and the coulombic efficiency for hydrogen peroxide formation was only 10–20%, but in the presence of Cl − and Br − ions this efficiency increased to 50–60%. Current-potential curves indicated that overpotential of the oxygen reduction at the platinum electrodes increased in the presence of Ba 2+, Sr 2+, and Ca 2+ ions in I M NaOH, and in the presence of Cl − and Br − ions in I NH 2SO 4. Catalytic activity of the platinum electrodes for hydrogen peroxide decomposition in I M NaOH and I N H 2S0 4 with and without the addition of the anions and cations was also measured. These results are discussed on the basis of ion-exchange adsorption of cations and anions on. the oxide-covered platinum electrodes. It is concluded that the oxygen reduction on platinum is not one process, but at least two processes including O 2+ H 2O + 2 e → OOH − +OH − in alkaline solution and O 2 + 2 H + + 2 e → H 2O 2 in acid solution, and the proportion of hydrogen peroxide formation depends on the surface condition of the electrode, such as presence of oxide film and/or adsorption of anions and cations. Oxide films and/or adsorbed oxygen which exist on the platinum electrodes were summarized and a voltammetric study of a special oxide film formed on an excessively anodized platinum, electrode was described. Overpotential of oxygen evolution on other metals and graphite as well as on Pt in I M NaOH increased by l59̃0 mV when Ba 2+, Sr 2+, or Ca 2+ ions were added to the electrolyte.