During the O-2 reduction reaction (ORR) at the Pt cathode of a polymer electrolyte fuel cell (PEFC), a slight amount of H2O2 is produced as a by-product. The generated H2O2 degrades the polymer electrolyte membrane and Pt electrode due to its corrosive nature. It is reported that a relationship exists between the amount of H2O2 and an amount of ionomer used in an electrocatalyst layer. To clarify the relationship, we employed a Pt microdisk electrode so that the Nafion-coated thickness could be uniformly controlled. The H2O2 amount was then investigated with respect to the overcoated Nafion thickness. The generated H2O2 at the Nafion-coated Pt generator during the ORR is oxidized at a Pt detector of the scanning electrochemical microscope. As a result, the H2O2 detection current increased with the increasing Nation thickness up to 0.16 mu m. The current then decreased in the thicker Nafion region. The H2O2 increase at the thin Nation/Pt can be explained by a decrease in the number of Pt-Pt pair-sites for the ORR. For the H2O2 decrease in the thicker region, a low diffusion coefficient of H2O2 in the Nafion layer makes it difficult to spread out. (C) 2013 Elsevier Ltd. All rights reserved.