The structure of sulfur adlayer and the formation process of Cu 2 S layer on Cu͑111͒ were investigated using in situ scanning tunneling microscopy ͑STM͒ in alkaline solution. In situ STM revealed two different structures of the sulfur adlayer, () ϫ ))R30°and (19 ϫ 19), in the double-layer potential region. The () ϫ ))R30°structure observed at negative potentials was transformed into (19 ϫ 19) upon sweeping the potential in the positive direction. The (19 ϫ 19) structure consisted of well-ordered triangular domains separated by boundaries running along the ͗110͘ direction. In each triangular domain of (19 ϫ 19), sulfur atoms were found to form a () ϫ ))R30°structure. Characteristic hexagonal rings consisting of six sulfur atoms were observed at intersections of the domain boundaries of (19 ϫ 19). At anodic potentials, layers of Cu 2 S were epitaxially formed on Cu͑111͒, on which a (ͱ7 ϫ ͱ7)R19.1°structure was observed.Sulfur atoms are known to be strongly adsorbed on metal surfaces and to play important roles in catalysis and many electrochemical reactions such as corrosion. Adlayer structures of S on ͑111͒ surfaces of Pt, 1 Rh, 2 Ni, 3 Pd, 4,5 and Cu 6-12 have been extensively studied in ultrahigh vacuum ͑UHV͒ environment. These studies revealed that the S adlayer forms various ordered structures on ͑111͒ metal surfaces depending on the coverage. Among various metal surfaces, the adsorption of S on Cu͑111͒ has been most intensively studied since S atoms act as a strong poison to Cu-based catalysis, such as the water-gas shift reaction. 9,10 Moreover, the reaction between Cu and S results in the formation of copper sulfide, 6-12 which is a compound semiconductor used in optical devices such as solar cells. 13,14 Adlayer structures of S on Cu͑111͒ surface have been investigated mainly by using scanning tunneling microscopy ͑STM͒ and low-energy electron diffraction ͑LEED͒ under UHV. At a very low S coverage at a low temperature, an STM image was obtained that could be described as (ͱ43 ϫ ͱ43)R Ϯ 7.5°. 12 At a coverage of one-third, it was found by using LEED that S formed a simple () ϫ ))R30°structure. 6 STM studies revealed that when the S coverage was increased, the adlayer structure was changed into ( Ϫ1 4 4 1 ). 7-9 At the saturation coverage, a (ͱ7 ϫ ͱ7)R19.1°structurewas observed by LEED 6,10 as well as STM. 7-9 The ( Ϫ1 4 4 1 ) and the (ͱ7 ϫ ͱ7)R19.1°adlayers were also studied using other techniques, such as surface X-ray diffraction 11 and Auger electron spectroscopy. 10 These studies indicated that the ( Ϫ1 4 4 1 ) and(ͱ7 ϫ ͱ7)R19.1°structures were not simply attributable to the formation of adlayers of S on Cu͑111͒, and that the formation of copper sulfide compounds from a chemical reaction between Cu and S atoms must also be taken into account. According to the previous studies in UHV, the adlayer structure of S on Cu͑111͒ seems to be strongly affected by its coverage. In solution, adlayer structures of specifically adsorbed anions have long been investigated, and several potential-dependent structures ha...