Using in-situ X-ray diffraction, we have studied the underpotential deposition of Ph onto a Pt(001) electrode both in pure perchloric acid and in the presence of bromide anions. In pure perchloric acid, the Pb adlayer forms monolayer-high islands which fully cover the Pt surface but have no long range order in the surface plane. The presence of strongly adsorbed Br atoms at step edges on the Pt(001) surface and the competitive nature of the Pb-Br interaction cause a rapid exchange of Pb and Br on the Pt terraces, the Pb then forming a strongly bonded c(2 x 2) adlayer.