The effects of the atmospheric trace species (SO2, Cl -, NH3) on the formation of H2O2 accompanying the O2 reduction reaction (ORR) have been elucidated in 0.1 M HClO4 using rotating ring-disk Pt electrode methodology. The potential at which the ORR starts shifts to the negative direction and the amount of formed H2O2 increases concomitantly as the amount of added SO2 or CI- increases, although Cl - less affects. The extent of the H2O2 formation is correlated to the coverage of SO2 or Cl- which was estimated from the decrease in the area of a hydrogen desorption peaks. This suggests that the adsorption of SO2 or Cl- on Pt reduces the number of the adjacent adsorption sites necessary for breaking of the O-O bond and then enhances the production of. No effect on the ORR activity and the H2O2 formation was observed for NH3.