Abstract Single crystal Cu(110) electrodes have been investigated by using in situ scanning tunneling microscopy (STM) in various solutions containing halide ions (I −, Br − and Cl −). A Cu(110)–(1×1) structure was observed consistently on Cu(110) in pure HClO 4 solution. Structures of halide adlayers were revealed with atomic resolution. The results showed that both iodide and bromide formed identical c(2×2) adlattices, while a reconstructed structure appeared immediately after chloride adsorption. Two copper atomic rows added to the (1×1) surface in the 〈001〉 direction with every two rows absent, resulting in a (4×1) reconstructed structure. The orientation of the added rows is parallel to the 〈001〉 direction of the Cu(110) substrate. Chloride adsorbed on the reconstructed Cu(110) surface with the same (4×1) registry. It was found that the anodic dissolution of Cu(110) in HCl occurred preferentially along the 〈001〉 direction.