The underpotential deposition (UPD) of copper on a Pt(111) electrode and the influence of gas coadsorbates, i.e. CO and CO₂, on the thus deposited copper layer were studied in a 0.1 M HClO₄ electrolyte by means of EC-STM. By UPD, an atomically flat Cu layer is formed, which exhibits a pseudomorphic (1 × 1) structure. However, it contains several point defects due to which its total coverage is less than a monolayer, in agreement with the measured charge density in the CV curves. Upon exposure to a CO-saturated solution the pseudomorphic structure collapses to a coalescent structure with many vacancy islands. This phase transition is induced by the preferential binding of CO to the Pt(111) surface. In contrast, CO₂, which binds stronger to copper, does not affect the pseudomorphic structure of the Cu layer.