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Doped carbon-based systems have been extensively studied over the past decade as active electrocatalysts for both the two-electron (2e^–) and four-electron (4e^–) oxygen reduction reactions (ORRs). However, the mechanisms for ORR are generally poorly understood. Here, we report an extensive experimental and first-principles theoretical study of the ORR at nitrogen-doped reduced graphene oxide (NrGO). We synthesize three distinct NrGO catalysts and investigate their chemical and structural properties in detail via X-ray photoelectron spectroscopy, infrared and Raman spectroscopies, high-resolution transmission electron microscopy, and thin-film electrical conductivity. ORR experiments include the pH dependences of 2e^– versus 4e^– ORR selectivity, ORR onset potentials, Tafel slopes, and H/D kinetic isotope effects. These experiments show very different ORR behavior for the three catalysts, in terms of …