The solution condition producing the discontinuity for each halide is describable by [Pb2 + ][X-]2 = constant. This effect, not observed in fluoride solutions, is inconsistent with simple anion induced adsorption and is interpreted as a two-dimensional precipitation forming the surface phase PbX2. Chronopotentiometric measurements of the inhibiting properties of adsorbed PbX2 on Hg(ll) reduction and Hg(0), Cd(0), and Zn(0) oxidation are presented to show that the inhibition is the same for electrodeward or solutionward passage through the adsorbed layer. The sudden appearance of the inhibition with increasing lead concentration is independent of the reaction being inhibited and appears to originate in a structural or crystal-perfecting change in the PbX2 surface phase.Data on the adsorption, on mercury electrodes, of white metal cations from aqueous solutions of simple anionic ligands is now fairly extensive. We have available surface excess data for the adsorptions of Cd(II) (1, 2), Hg(II) (3), and Pb(II) (3, 4) from iodide medium; Cd(II) (2), Hg(II) (3), and Pb(II) (3) from bromide medium; Pb(II) (5,6) from chloride medium, Cd(II) (7), Hg(II) (3, 8), Pb(II) (3, 4), In(III) (9), and Zn(II) (6, 10) from thiocyanate medium, Cd(II) (11) from thiosulfate medium, and Cd(II) (12) and Zn(II) (12) from azide medium. Accumulation of these data has been materially aided by development of the double potential step chronocoulometric technique (13) as used with minicomputer control and data acquisition ( 14), the current approach of choice.