Oxygen reduction reaction (ORR) activity and H2O2 formation at Nafion-coated film electrodes of bulk-Pt and Pt nanoparticles dispersed on carbon black (Pt/CB) were investigated in 0.1 M HClO4 solution at 30 to 110 C by using a channel flow double electrode method. We have found that the apparent rate constants k(app) (per real Pt active surface area) for the ORR at bulk-Pt (with and without Nafion-coating) and Nafion-coated Pt/CB ( 19.3 and 46.7 wt % Pt, dPt) 2.6 to 2.7 nm) thin-film electrodes were in beautiful agreement with each other in the operation conditions of polymer electrolyte fuel cells (PEFCs), i.e., 30-110 degrees C and ca. 0.7 to 0.8 V vs RHE. The H2O2 yield was 0.6-1.0% at 0.7-0.8 V on all Nafion-coated Pt/CB and bulk-Pt and irrespective of Pt-loading level and temperature. Nafion coating was pointed out to be a major factor for the H2O2 formation on Pt catalysts modifying the surface property, because H2O2 production was not detected at the bulk-Pt electrode without Nafion coating.