Using electrochemical measurements and in situ scanning tunneling microscopy (STM), we confirm the process of Ag growth and the overlayer structures of Ag underpotential deposition (upd) on Au(111) in the presence or absence of chloride anions. There are four distinct structures of the Ag adlayer on Au(111) in the presence of chloride. Adlayer structures (root3 x root3)R30degrees, (root19 x root19)R37degrees, (root13 x root13)R46degrees, and (6 x 6) are formed as a function of potential, and these structures are clearly matched to the features in a cyclic voltammogram. Compared with the (4 x 4) structure of Ag upd on Au(1 11) in a pure 0.1 M HClO4 solution, overlayer structures of Ag on Au(111) in a chloride-containing solution are more loosely packed. Therefore, we conclude that the structures of the Ag overlayer on Au(111) strongly interact with chloride anions, suggesting the bilayer structure is composed of both Ag and Cl.