Monodisperse Pt nanoparticles supported on a graphitized carbon black (GCB; 150 m(2) g(-1)), which exhibits higher resistance to carbon corrosion than a conventional high-surface-area carbon black (CB; 800 m(2) g(-1)), were prepared by the nanocapsule method. At the practical temperature of 65 degrees C for polymer electrolyte fuel cells, three kinds of 50 wt.%-Pt loaded catalysts [our nanocapsule Pt/GCB (n-Pt/GCB), a commercial Pt/GCB (c-Pt/GCB), and a commercial Pt/CB (c-Pt/CB)] were subjected to a durability test by a standard potential step protocol (E = 0.9 V <-> 1.3 V vs. RHE, holding 30 s at each E) in N-2-purged 0.1 M HClO4 solution. After a given number of the potential step cycles N, changes in the electrochemically active area (ECA), the kinetically-controlled mass activities (MA) for the oxygen reduction reaction, and H2O2 yield at Nafion-coated film electrodes of these catalysts were examined at 65 degrees C by the multichannel flow double electrode (M-CFDE) method. The N value at which the slowly decreasing MA(0.8V) for n-Pt/GCB reached 1/2 of the initial level was 19 times larger than that for c-Pt/GCB and 156 times larger than that for c-Pt/CB. The value of H2O2 yield at n-Pt/GCB was the lowest among the catalysts examined. The highly uniform distribution and monodisperse nature of the Pt particles on the GCB support are the most essential factors for the excellent durability of n-Pt/GCB. (C) 2012 Elsevier B.V. All rights reserved.