A cylindrical two-dimensional model based on the Nernst-Planck equations, the Navier-Stokes equation, and the continuity equation is used to simulate the oxygen reduction reaction in 0.5 M H2SO4 at a rotating ring disk electrode. Concentration distributions and a potential profile are obtained as a function of the axial and radial distances from the center of the electrode surface. Polarization curves are simulated to interpret experimental results by studying various reaction mechanisms, i.e., the four-electron-transfer reduction of oxygen, the two-electron-transfer reduction of oxygen, a combination of the above two reactions, mechanisms with reduction of peroxide to water, and/or the heterogeneous chemical decomposition of peroxide. Special attention is devoted to the effect of peroxide. (c) 2007 The Electrochemical Society.