The structure of sulfur adlayer and the formation process of Cu2S layer on Cu(111) were investigated using in situ scanning tunneling microscopy (STM) in alkaline solution. In situ STM revealed two different structures of the sulfur adlayer, (root3 x R30degrees and (19 x 19), in the double-layer potential region. The (root3 x root3) R30degrees structure observed at negative potentials was transformed into (19 3 19) upon sweeping the potential in the positive direction. The (19 3 19) structure consisted of well-ordered triangular domains separated by boundaries running along the [110] direction. In each triangular domain of (19 x 19), sulfur atoms were found to form a (root3 x root3) R30degrees structure. Characteristic hexagonal rings consisting of six sulfur atoms were observed at intersections of the domain boundaries of (19 x 19). At anodic potentials, layers of Cu2S were epitaxially formed on Cu(111), on which a (root7 x root7) R19.1degrees structure was observed. (C) 2003 The Electrochemical Society.