Colloidal Pt prepared by an ethylene glycol reduction method was deposited onto Ketjen black carbon supports functionalized with (0, 3.2, 7.1, and 15.9wt%) 11-silicotungstic acid (Pt/SiW11-C). Electrochemical characterization of the catalysts was performed using rotating disk electrodes (RDE) in 0.1 M HClO4 electrolyte. XRD and TEM showed smaller crystallite size and more uniform deposition of Pt nanoparticles for Pt/SiW11-C catalysts. respectively. A maximum in the ORR mass activity of 373 mA/mg(Pt) was observed for the 3.2wt% SiW11 catalyst. an 18% improvement over Pt/C. An increase in the electrochemical area (ECA) due to lower Pt particle size and more narrow size distribution is attributed to providing the mass activity enhancement. After 30,000 durability cycles in the potential range 0.6-1.0 V. Pt/SiW11-C showed less Pt particle growth (TEM), and a factor of 1.4 improvement in terms of mass activity retention. After 6.000 durability cycles in the potential range 1.0-1.6 V, Pt/SiW11-C showed a factor of 2 increase in mass activity retention compared to Pt/C. The improvement is attributed to a slower rate of carbon corrosion. (C) 2012 The Electrochemical Society. [DOI: 10.1149/2.068212jes] All rights reserved.