The structure and chemical composition of electrosynthesized ZnS thin films on Au(111) substrates was studied by scanning tunneling microscopy (STM) and X-ray photoelectron spectroscopy (XPS). The films mere grown by alternating underpotential deposition and oxidative adsorption cycles of S and Zn from solution precursors (electrochemical atomic layer epitaxy, EC-ALE). Oxidative adsorption of the first layer of S results in a (root 3 x root 3)R30 degrees adlattice on Au(111). In the initial Zn monolayer, XPS indicates the presence of SO42-derived from the supporting electrolyte. The coadsorbed SO42-can be chemically or electrochemically displaced by H2S(aq) or HS(aq)-, leading to the formation of ZnS. Atomically resolved STM images show the ZnS/Au(lll) monolayer to be 6-fold symmetric with an interatomic spacing of 0.37 +/- 0.01 nm. The structure of the deposit on the micrometer scale was also investigated by STM. The first complete EC-ALE cycle results in the formation of nanocrystallites of ZnS randomly distributed across Au(111) terraces. The average diameter of the crystallites is 10 +/- 5 nm, and the apparent coverage of ZnS is 0.38.