This work describes the study of underpotential deposition (upd) of Pb on polycrystalline platinum in 0.2 M HClO4 solutions using cyclic voltammetry and rotating ring-disk electrode techniques. In such conditions, a full monolayer of upd Pb corresponds to a coverage of 0.41. The adsorption of lead inhibits the adsorption of hydrogen in a non-selective way. This behavior was explained through the comparison between the radius of the ad-atom and the substrate. It was calculated that each Pb ad-atom occupies two sites in the surface. Additionally it is shown that the presence of upd Pb significantly inhibits the oxygen reduction reaction. This effect was associated to the formation of H2O2 as the final product, instead of H2O.