Oxygen reduction in acidic electrolyte solution has been studied at various Au/C catalysts with different support materials and different gold particle sizes. The peak potential in cyclic voltammogramms (CVs) during oxygen reduction depends on the potential applied to the catalyst during electrochemical pretreatment. This effect is attributed to a shift of oxide reduction on the nanoparticles to more negative potentials compared to bulk gold, as can be seen from CV. Quantitative assessment of the oxygen reduction activity has been carried out with rotating disc electrode (RDE) measurements. A catalyst with a gold loading of 0.5 wt% gold turned out to be the most active in terms of mass specific activity. However, the surface specific activity was neither dependent on the particle size in the range 2.7-42.3 nm nor on the support material. It has been found the average number of electrons transferred per oxygen molecule is two to three, independent on gold particle size. (C) 2008 Elsevier B.V. All rights reserved.