The impact of 3M's nanostructured thin film electrocatalyst loading on rotating ring-disk electrode (RRDE) experiments for the oxygen reduction reaction (ORR) in a 0.1 M HClO4 solution has been studied. The electrocatalysts were prepared by sputtering Pt onto support whiskers and have been reported to exhibit one of the best specific activities of any ORR catalyst thus far. However, a dramatic increase in H2O2 release was observed as the catalyst loading (number of catalyzed whiskers per unit area) on the glassy carbon tip of RRDE was decreased below 20 mu g cm(-2), confirming some earlier observations reported by our group and other research groups with some other types of catalysts. These observations suggest that oxygen reduction occurs through a H2O2 intermediate, and if the catalyst nanoparticles are sparsely distributed, the produced H2O2 cannot be efficiently reduced to H2O before it escapes. Similar observations have been made for Se/Ru/C and Fe-N-C catalysts. (C) 2008 The Electrochemical Society.