The dependence of oxygen reduction reaction (ORR) activity on the loading level of Pt electrocatalysts highly dispersed on carbon black (Pt/CB) has been investigated. We present a standard method for the evaluation of ORR activity at Pt/CB by a rotating disk electrode (RDE), in which Pt/CB powder was attached on a glassy carbon (GC) disk uniformly, followed by coating a thin Nafion film. We describe the optimized protocols for preparing a suspension of the Pt/CB in an ethanol aqueous solution, controlling vapor pressure during drying process, and coating very thin (0.05 mu m) Nafion film. The area-specific activity for the ORR in air-saturated 0.1 M HClO4 electrolyte solution was found not to be influenced by the Pt-loading level on CB from 19.2 to 63.2 wt%, when the amount of Pt/CB on GC and the dispersion state were optimized. (C) 2005 Elsevier B.V. All rights reserved.