The underpotential deposition (UPD) of Cu and Ag from sulphuric acid solutions was studied on polycrystalline Au surfaces by cyclic voltammetry at stationary and rotating ring-disk electrodes. Since the voltammetric profiles also include the surface oxide/reduction contributions, the charges were also calculated by means of the collection technique of the rotating ring-disk system. The charge values obtained from cyclic voltammetry at stationary and rotating ring-disk electrodes were 405 and 386 mu C cm(-2), respect ively, for the oxidation of a complete monolayer of UPD Cu. The proximity of these values suggests epitaxial deposition in a two-electron reduction process. On the other hand, the charge values found for UPD Ag were 108 and 115 mu C cm(-2), which corresponds to the amount required for the oxidation of only half a monolayer of Ag. Since the atomic radii of the three atoms involved (Cu, Ag and Au) are very similar, the differences in the deposition models were associated with the co-adsorption of anions (sulphate or hydroxide ion) which hinders the formation of a full monolayer of Ag on polycrystalline Au surface.