Electrochemical reactivity regarding hydrogen reduction was studied at epitaxially grown Pd monolayers and sub-monolayers on Au(1 1 1) in 0.1 M HClO4 solution. The rate of hydrogen evolution increases with decreasing numbers of layers, and it is considerably higher for sub-monolayers, i.e. the fewer Pd islands are on the surface the higher is the catalytic activity. No clear dependence of the reactivity on the ratio of Pd edge atoms to terrace atoms was found. Possible mechanisms explaining the experimental results are discussed. (c) 2007 Published by Elsevier Ltd.