Electrochemical and ultrahigh vacuum measurements are presented for the underpotential deposition (UPD) of Pb on Cu(100) in the presence of chloride ions. The chemistry is very similar to that for the (111) surface studied previously by the same methods. UPD Pb forms a compact layer of fully discharged atoms having essentially the same density as that in the (100) plane of bulk Pb. The deposition appears to occur via a single process, the nucleation and growth of Pb islands, but other types of growth modes cannot be excluded. The presence of chloride ion in the electrolyte enhances the kinetics of Pb deposition in both the underpotential and overpotential regions and results in a approximate to 0.1 V negative shift in the mean potential for UPD versus that in chloride-free perchloric acid. This shift can be accounted for in a simple thermodynamic model for the total Cu/Pb/Cl system. The only real difference in Pb UPD on the two different Cu crystal faces was the absence of an ordered structure for the compact layer on Cu(100), versus the ordered structure observed on Cu(111). This difference appears to be related to a lower mobility of Pb adatoms due to the greater atomic corrugation of the (100) surface.