Cu underpotential deposition on Au(110), in pure and Cl- containing H2SO4 solution, was investigated by in situ STM. In the absence of Cl- the general surface topography and the (1 x 1) structure of the nonreconstructed surface are maintained over the entire upd potential range. This is interpreted as formation of a (1 x 1) Cu adlayer, with weak and presumably slightly repulsive adsorbate-adsorbate interactions. In Cl- containing electrolyte a (2 x 1) structure is reversibly formed at potentials below 0.27 V-sce, coincident with a sharp peak in the cyclic voltammograms and the appearance of anisotropic, [001] oriented Au bilayer islands and monolayer holes on the previously flat terraces. A model based on the CuCl(111) bilayer is suggested for the (2 x 1) structure and compared to adlayer structures on Au(111) and (100). The roughening is interpreted as anion induced surface alloying.