Surface X-ray scattering (SXS) in transmission geometry provides a valuable tool for ill Situ Structural studies of electrochemical interfaces Under reaction conditions, as illustrated here for homoepitaxial electrodeposition oil Au(100) and Au(111) electrodes. Employing diffusion-limited deposition conditions to separate the effects of potential and deposition rate, a Mutual interaction between the interface structure and the growth behavior is found. Time-dependent SXS measurements during, An(100) homoepitaxy show with decreasing potential transitions from step flow to layer-by-layer-growth, then to multiayer growth, and finally back to layer-by-layer growth. This complex growth behavior can be explained within the framework of kinetic growth theory by the effect of potential, Cl adsorbates and the Au surface structure, specifically the presence of the surface reconstruction, Oil the Au surface mobility. Conversely, the electrodeposition process influences the structure of the reconstructed Au surface, as illustrated for Au(111), where, a significant deposition-induced compression of the An surface layer as compared to Au(111) surfaces under Ultrahisdi vacuum conditions or in Au-free electrolyte is found. This compression increases towards more negative potentials, which may be explained by a release of potential-induced Surface Stress. (c) 2007 Elsevier Ltd. All rights reserved.