The stability of underpotentially deposited (UPD) Pb layer on Cu(111) is investigated by conventional electrochemical techniques in perchlorate solution at open circuit potential (OCP). In situ scanning tunneling microscopy (STM) is employed to monitor and ascertain structural and morphological changes at characteristic potentials. A corrosion-like mechanism associated with UPD layer stripping powered by reduction processes is found to operate in the system of interest in absence of potential control. OCP transients suggest strong dependence of the Pb layer stability upon the concentration of oxidizing agents, such as oxygen and/or nitrate ions, present in the solution. It is found that the increase of the oxidizing agent concentration results in a proportional decrease of the Pb UPD layer stripping time at OCP. The concentration of the dissolved oxygen is found to affect the UPD layer behavior in the entire range of underpotentials in accordance with the strong affinity of the Pb2+/Cu(111) system to the oxygen reduction reaction (ORR). In contrast to oxygen, nitrate ions appear to play active role only in the potential range positive to the UPD peaks where mostly bare Cu surface is in contact with the solution. Specifically adsorbing Cl− ions are examined as a possible inhibitor of the reduction processes operating in the Pb underpotential range. Concentrations of Cl− ions as low as 1×10−4 M are found to stabilize the Pb UPD layer by a factor of 2.5.