A rotating platinum disk electrode was used to study oxygen reduction over the entire pH range and to compare the kinetics in acid and alkaline solutions. In both solutions two Tafel regions are confirmed. At low cd's ∂ V/∂ln i = − RT/F and at high cd's it is −2 RT/F. In the low cd region, the reaction order with respect to H 3O + is 3 2 in acid, and 1 2 in alkaline solutions. For the high cd region the reaction orders are 1 and 0 in acid and alkaline solutions, respectively. Transition of the kinetics from the low to the high cd region, as well as the fractional reaction orders at low cd's, are interpreted in terms of the first charge transfer step as rate determining in both cd regions but under Temkin adsorption conditions in the low, and Lagmuirian conditions in the high cd region. Coverages, θ, with oxygen intermediates are also determined as a function of potential and pH. At all pH's, substantial coverages are observed only at potentials in the low cd region. The usual Tafel slope of −120 mV for the first charge transfer step as rate determining is not observed at low cd's since it is modified by the dependence of activation energy on θ, and hence on V. At high cd, θ is low and ineffective and the usual slope for the first charge transfer step is observed. At all pH's the transition from low to high cd's occurs at potentials, V T , that decrease 60 mV as pH increases one unit. Dependence of V T and i T on pH is discussed. Fractional reaction orders at low cd's are also related to θ. Real reaction orders with respect to H 3O + are modified by the dependence of activation energy on θ, and hence on pH. The change of the kinetics from the acid to the alkaline region is discussed. The range of cd's accessible to measurements is limited at both high and low cd's. At high cd's it is limited by the supply of oxygen and/or H 3O + from the solutions. At low cd's, V—log i relationship extends only to a rest potential which decreases 60 mV as pH increases one unit. The limits of measurements and the nature of the rest potential are discussed.