Adlayers of sulfur (S) and thiocyanate (SCN - ) on Cu(111) were investigated by using in situ scanning tunneling microscopy in alkaline solution. Two different structures of S adlayers, (√3x√3)R30 o and (19x19), were revealed in the double-layer potential region. The (√3x√3)R30 o structure observed at negative potentials was transformed into (19x19) on sweeping the potential in the positive direction. For SCN - on Cu(111), the (√3x√3)R30 o 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 (19x19) structure of the S adlayer.