Adlayers of sulfur (S) and thiocyanate (SCN-) on Cu(I 1 1) were investigated by using in situ scanning tunneling microscopy in alkaline solution. Two different structures of S adlayers, (root3 x root3)R30degrees and (19 x 19), were revealed in the double-layer potential region. The (root3 x root3)R30degrees structure observed at negative potentials was transformed into (19 x 19) on sweeping the potential in the positive direction. For SCN- on Cu(I 1 1), the (root3 x root3)R30degrees structure was also observed at negative potentials. At positive potentials, a structural change was observed also for the SCN adlayer; however, the adlayer structure was much distorted compared with the (19 x 19) structure of the S adlayer. (C)0 2003 Elsevier Science B.V. All rights reserved.