The underpotential deposition (upd) of Pb onto Pt(lll) in solution containing Br- was examined by classical RRDP(111)E electrochemical methods in combination with in situ surface X-ray scattering measurements. The results show that Pb upd onto Pt(lll) in the presence of bromide is a competitive process with the total amount of Pb deposited at underpotential being only ca. 0.51 +/- 5% ML, less than the amount needed to form a close-packed monolayer, 0.63 ML (1 ML equals 1 Pb atom per Pt surface atom). Deposition of Pb adatoms is accompanied by desorption of about one-half of the bromide adsorbed on the Pt surface at higher potentials, leaving co-adsorbed bromide both in and on-top-of the adlayer. In the potential region where the ratio of co-adsorbed species is close to one (E approximate to OV) there is the slow formation (over a period of ca. 2h) of an ordered surface alloy, which has a p(2 x 2) unit cell. No other ordered structures of Pb are observed in the underpotential region.