The gold(110) single crystal exhibits a (1 × 2) surface structure when clean, but when immersed in 0.01 M HClO4 under potential control the structure has been found to change. Throughout the double-layer region the structure remains as (1 × 2) but when the potential is increased above 0.9–1.0 V vs. Ag/AgCl (the pre-oxidation region) the surface structure changes to a centred rectangular structure. This structure forms reproducibly but has limited stability. After the initial oxidation, the structure prevails until 1.2–1.3 V, whereupon oxidation occurs on a larger scale and the surface structure becomes (1 × 1). These structures return to (1 × 2) after reduction of the oxide, proving that the structural changes are reversible. X-ray photoelectron spectroscopy of these regions has shown a difference in the chemical state of the oxygen. Adsorbed perchlorate ion is initially the only oxygen component but as the potential increases the peak shifts into the hydroxide region and finally splits into a hydroxide peak and a metal oxide peak. This study indicates a new way of looking at the structure and composition of the gold surface and provides insight into the nature of the interaction between the surface and the solution.