Density-functional calculations were performed to investigate oxygen reduction reactions on a ZrO2 ((1) over bar 1 1) surface. We found that a spontaneous bond breaking in HOOH suppresses the termination of the reaction at 2e reduction, which indicates that at least O-2 can be reduced to OH on the surface through molecular dynamics simulation. The possibility of OH poisoning was suggested after calculating the equilibrium potential of each reduction step. (C) 2008 Elsevier B. V. All rights reserved.