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 x 1) structure was observed consistently on Cu(110) in pure HClO4 solution. Structures of halide adlayers were revealed with atomic resolution. The results showed that both iodide and bromide formed identical c(2 x 2) adlattices, while a reconstructed structure appeared immediately after chloride adsorption. Two copper atomic rows added to the (1 x 1) surface in the [001] direction with every two rows absent, resulting in a (4 x 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 x 1) registry. It was found that the anodic dissolution of Cu(110) in HCl occurred preferentially along the [001] direction.