Density-functional calculations were performed to examine oxygen reduction reactions (ORRs) on N-doped graphene sheets. We found that O-2 adsorption becomes energetically favorable as the number of N around a C=C bond increases. Pathways for both 4e and 2e reductions were identified. The possibility of O poisoning was suggested after calculating the reversible potential of each reduction step. (C) 2009 Elsevier B.V. All rights reserved.