Underpotential deposition (UPD) of lead (Pb-upd) on Pt(111) in perchloric acid solution was studied using the rotation ring disk, electrode (RRDPt(111)E) method for measurements of the Pb2+ ion-specific flux. The total amount of Pb deposited underpotentially is 0.62 +/- 5% monolayer (1 monolayer = 1 Pb per Pt) equivalent to a close-packed monolayer of fully discharged Pb adatoms. The electrosorption valence, gamma(Pb), of Pb-upd is approximate to gamma(Pb) = 2, implying that two electrons per Pb-upd adatom are exchanged through the interface. In this work we showed that the formation of this close-packed monolayer of Pb-upd occurs through four distinctive voltammetric features : two major sharp peaks at approximate to 0.5 V (vs saturated calomel electrode) and 0.3 V and smaller reversible peaks at 0 V and -0.15 V. We concluded that the adsorption of OHad is shifted negatively by Pb-upd adatoms, with the shift attributed to induced adsorption of OHad onto Pt atoms neighboring the Pb-upd adatoms due to lowering of the local point of zero charge (pcz) by the Pb-upd. Consequently, the pH dependence of the Pb UPD peak at 0.3 V appears entirely through a pH-dependent adsorption of OHad onto the Pt(111) surface modified by Pb-upd adatoms. The pH independence of the peak at 0.5 V is consistent with the Pb2+ ion-flux measurements that the main process associated with this pseudocapacitance is Pb UPD. The smaller peaks observed at lower potentials are probably associated with a final deposition of Pb-upd adatoms along step edges and compacting of adatoms on the (111)-terraces to form an ordered (3 x root 3) close-packed structure.