The effect of the mass transport coefficient of reactant and product species during the oxygen reduction reaction, orr, on platinum in an acidic electrolyte has been experimentally examined and kinetically modeled. By using carbon electrodes having electroactive radii on the nanometer scale it is possible to produce single Pt particles having effective radii ranging from several micrometers to several tens of nanometers. As the mass transport coefficient is directly related to the size of these platinum particles, it is possible to examine the effect of mass transport on the orr in regions inaccessible to other experimental techniques. At the smallest of these Pt particles, mass transport coefficients equivalent to a rotating disk electrode at rotation rates (omega) of greater than 108 rpm are obtainable. Under low mass transport conditions equivalent to those obtainable using the normal rotating disk technique (i.e, omega < 10 000 rpm), oxygen reduction is seen to proceed via a four-electron reduction to water as has been reported in the general literature. Under high mass transport conditions about 75% of reactant oxygen molecules are reduced to water with the balance being only reduced as far as hydrogen peroxide. The production of peroxide which this result implies may be an important aspect within the cathode catalyst layer of solid polymer electrolyte fuel cells, as these layers are inherently designed to provide high mass transport coefficients. The oxygen reduction reaction on single catalyst particles is modeled according to the parallel reaction mechanism originally introduced by Wroblowa et al. [Wroblowa, H. S.; Pan, Y. C.; Razumney, J. J. Electroanal. Chem. 1976, 69, 195]. A general expression is derived to predict the effects of the mass transport rate, surface blockage, and potential on the effective electron-transfer number, n(eff), which reflects the average number of electrons produced during the reduction of each dioxygen molecule. It is shown that a pure series (or indirect) reaction mechanism for the four-electron reduction of oxygen on Pt electrodes in sulfuric acid solution is consistent with the experimental results. The kinetics of the orr is analyzed using both Tafel plots and the half-wave potential method. The kinetic parameters extracted from the half-wave potential method are in very good agreement with those extrapolated from the Tafel curves with a -120 mV per decade slope. The complexities involved in the orr kinetics are discussed according to the results obtained on these small single-particle electrodes. Specifically, the effect of the double-layer structure and the role of anion adsorption are considered. It is argued that the electrocatalytic reduction of oxygen involves inner-sphere electron-transfer steps and that its kinetics are affected by the potential profile within the compact double layer, especially inside the inner Helmholtz plane (IHP). Anion adsorption may perturb the orr in a way much more complex than simply blocking the surface sites and may significantly change the potential near the IHP, thereby changing the effective driving potential that the reactant and reaction intermediates experience. This may be partly responsible for the variable apparent values of the transfer coefficient of the orr in different potential regions. A new mechanism for the size effects of catalyst nanoparticles on their electrocatalytic properties toward oxygen reduction is proposed in terms of the particle size tunable structure of the double layer.