The effect of catalyst loading levels on the oxygen reduction reaction (ORR) selectivity for water formation on both nitrogen-containing nanostructured carbon (CNx) and commercial Pt/Vulcan carbon (VC) catalysts was examined using the rotating ring disk electrode (RRDE) method. Catalyst loadings from 142 to 1420 mg/cm(2) were studied. It was found that catalyst loading played a significant role in determining the selectivity for water formation, with lower loading levels showing a much higher selectivity for H2O2. The phenomenon was probed by examining the experimental collection efficiencies of the RRDE for each loading level and the catalyst thickness with scanning electron microscopy. The difference between experimental and theoretical collection efficiencies did not contribute significantly to the change in selectivity, although catalyst thickness did. It appears that a multistep mechanism where H2O2 is an intermediate can account for this behavior. More of this H2O2 intermediate can be detected when the reaction path length (catalyst thickness) is reduced. H2O2 electroreduction experiments showed the CNx materials to have the ability to catalyze further reduction of hydrogen peroxide to water, supporting the possibility of a two-step mechanism. (C) 2011 The Electrochemical Society. [DOI: 10.1149/1.3552944]