A rotating Pt3Co, thermally prepared alloy disc is used to study electrochemical oxygen reduction in alkaline solutions of different pH. In all solutions two Tafel regions are found: at low current densities partial derivativeE/partial derivativelog j = -2.3RT/F and at high current densities partial derivativeE/log j = -2.3 x 2RT/F. The reaction order with respect to OH- is confirmed to be -1/2 and 0 for low and high current density regions, respectively. The coverage, theta, with oxygen intermediates, determined by a programmed potentiodynamic method, is found to be a function of potential and pH. In all solutions, substantial coverages are observed only at potentials in the low current density region. It is found that the fractional reaction order at low current densities is related through the adsorption of oxygen species under Temkin conditions and the dependence of activation energy on theta and pH. (C) 2003 Elsevier Science B.V. All rights reserved.