The chloride adlayer on Au(1 0 0) electrodes in 0.01 M HCl solution was studied on the atomic scale by in situ high-speed scanning tunneling microscopy. In the potential regime 0-0.4 V-Ag/AgCl, a hitherto not reported defective c(2 x 2) structure was observed. In addition to characteristic straight domain boundaries oriented along the [0 1 0] and [0 1 1] directions of the Au substrate, unusual chain-like adsorbate structures were found. These defects can exhibit open structures with locally lower coverage as well as more close-packed arrangements, in which the adsorbates deviate from the lattice sites and have a 15% reduced nearest neighbor spacing. Dynamic fluctuations within the adlayer occur on the time scale of seconds, much slower than in Cl adlayers on Cu(1 00). The increase in defect density observed toward more positive potentials suggests a change of the adsorbate-adsorbate interactions with potential. Crown Copyright (C) 2013 Published by Elsevier Ltd. All rights reserved.