Au clusters (25 atoms) are up to 180 times more active for the hydrogen oxidation reaction (HOR) than bulk Au. They were created from catalytically inactive Au clusters (2-5 atoms) by electrochemical activation in the presence of Au ions. The HOR activity depends on the cluster's size and it is related to the fact that the position of the LUMO is below the redox potential for the HOR, while HOMO is below the H 1s-Au d antibonding resonance. (C) 2014 The Electrochemical Society. All rights reserved.