The hydrogen evolution reaction has been studied for ultrathin Pd overlayers of various thickness on Au(1 1 1) in 0.1 M H2SO4. A clear correlation of the electrocatalytic activity as expressed by the exchange current density and the binding energy of adsorbed hydrogen has been found. While hydrogen is bound strongest on the second Pd monolayer (ML), the respective catalytic activity is poorest for all the surface Structures under study. The exchange current density increases in the order 2 ML Pd < 1 ML Pd < bulk Pd (more than 2 ML). The electronic ligand effect, a geometric effect due to pseudomorphic growth and the surface defect density belong to the most crucial parameters in relations between Structure of the electrode surface and its electrocatalytic activity. The experimental results are supported by an excellent agreement with theoretical predictions. (c) 2008 Elsevier Ltd. All rights reserved.