The electrocatalytic properties of Au nanoparticles of mean size between 4.2 to 9.5 nm have been investigated for the oxygen reduction reaction (ORR). The particles were prepared on dispersed Vulcan XC-72R carbon black by reduction of a gold salt and by deposition of polymer stabilised gold sols. These were then attached to a glassy carbon disc electrode from their dispersion in a Nafion solution. The dependence on particle size of activity and selectivity for H(2)O(2) formation and the rate constants for oxygen and hydrogen peroxide reduction have been investigated using Rotating (Ring) Disc Electrode measurements. The electrocatalytic activity showed a maximum for a mean particle size of 5.7 nm and decreased significantly with particle size. The number of electrons exchanged per O(2) molecule increased from a value close to 2 to 3.4 as the potential was made more negative. The oxygen reduction selectivity for H(2)O(2) production was higher for mean particle sizes below 6 nm.