The underpotential deposition of submonolayer amounts of copper induces an enhanced adsorption of chloride and bromide on Pt(111) and is reflected in exceedingly sharp voltammetric peaks. The adsorbed anions are believed to be in contact with the platinum surface and in the vicinity of the electrodeposited copper. The electrosorption valency for chloride anions was estimated to be about 1. This effect of induced adsorption was not observed on Pt(100), pointing to the sensitivity of the process to the surface structure. The advantages of working with low concentrations of metals in underpotential deposition studies is illustrated.