Gold nanoparticles with narrow and controlled size distributions have been synthesized chemically and deposited onto a carbon support. Using the resulting gold on carbon (Au/C) catalysts, Au particle size effects on the kinetics of the oxygen reduction reaction (ORR) were analyzed in acidic media (0.5 M H2SO4). From rotating ring-disk electrode (RRDE) voltammetric studies, it was found that, for bulk gold, the number of electrons, n, involved in the ORR was nearly constant at potentials above -0.2 V. On the contrary, for the catalysts with diameters less than 10-15 nm, the value of n increased as the potential became more negative, and the highest value of n was obtained when the size of Au particles was less than 3 run. Those results showed that further reduction of H2O2 or direct 4-electron reduction of O-2 proceeded at relatively low overpotential on extremely small gold clusters. (C) 2009 Elsevier Ltd. All rights reserved.