The mechanism transition in the oxygen reduction reaction at the Pt-dispersed carbon (Pt/C) electrode was investigated in an oxygen-saturated 0.5 M H2SO4 solution. The reaction was monitored by acquiring data for Pt loading, Nation content and temperature by analyses of the rotating disk voltammograms and potentiostatic current transients (PCTs). From the shape of the cathodic PCTs and the dependence of the initial current density on the potential drop, it is suggested that oxygen reduction at the Pt/C electrode is controlled by the charge transfer at the electrode surface mixed with the oxygen diffusion in the solution below the value of the potential drop, Delta E-tr, needed for the occurrence of the mechanism transition, whereas oxygen reduction is purely governed by the oxygen diffusion in the solution above Delta Etr. In particular, it was noted that the value Of Delta E-tr remained nearly constant irrespective of the Pt loading and Nation content. On the other hand, the value of Delta E-tr decreased as temperature increased, which is ascribed to the fact that the contribution of the Cottrell current enhanced by temperature rise to the fall in Delta E-tr is overwhelmed by that contribution of the Butler-Volmer current increased. Consequently, it is concluded that it strongly depends upon the extrinsic parameters such as Pt loading, Nation content and temperature as well as the intrinsic parameters such as rate constant for interfacial reaction and oxygen diffusivity in the solution, which mechanism of the overall oxygen reduction reaction is operative. (c) 2007 Elsevier Ltd. All rights reserved.