The dynamic behavior of individual adsorbates at electrochemical interfaces was studied directly by in situ high-speed scanning tunneling microscopy, using sulfur adsorbed on Cu(100) electrodes in 0.01m HCl solution as an example. By dosing from diluted Na2S solutions Sad coverages of a few percent can be prepared, with the sulfur adsorbates occupying positions within the c(2×2) lattice of coadsorbed chloride. Sad tracer diffusion occurs via hopping between neighboring c(2×2) lattice sites at considerably higher rates than those of sulfur on Cu(100) under UHV conditions, indicating a pronounced influence of the electrochemical environment on the adsorbate surface dynamics. The diffusion barrier linearly increases by 0.5 eV per V with potential and is strongly affected by neighboring Sad and surface defects. The Sad-Sad interactions extend over ≈7ÅThey are repulsive between nearest-neighbor and attractive between next-nearest-neighbor sites, respectively, and result in significantly reduced diffusion barriers. Sad on the upper terrace side of steps are transiently trapped and exhibit lower diffusion rates, leading to the formation of small metastable p(2×2) domains. Attractive interactions between Sad and domain boundaries in the c(2×2) adlayer result in boundary pinning as well as transient trapping and enhanced diffusion of Sad along the boundary. © 2010 Wiley-VCH Verlag GmbH& Co. KGaA, Weinheim.