In situ Video STM was employed to study the formation mechanism and dynamics of the Cu checkerboard surface reconstruction induced by small coverages of adsorbed methyl thiolate on Cu(1 00) electrodes in 0.01 M HCl solution. The reconstruction process commences at potentials <=-0.60 V versus SCE by the formation of anti-phase shifted rows in the c(2 x 2) lattice of the chloride/thiolate coadsorbate layer, which are attributed to thiolates embedded into the Cu surface layer. These rows form a dense 2D network on the surface that exhibits pronounced fluctuations on the subsecond timescale, resulting in a highly disordered c(2 x 2) phase. This intermediate structure is replaced on time scales of minutes by the checkerboard reconstruction, in which the surface is covered by Cu adislands, consisting of small mobile islands of a slightly expanded c(2 x 2) structure. The dynamic observations support a stabilization of these adislands by thiolate, attached to the islands steps. (C) 2013 Elsevier B.V. All rights reserved.