Summary The electronic surface charge density of mercury electrodes has been measured as a function of their potentials in solutions of 1 mM MX, 1 mM MX+50 μM Na2SO4, and 1 mM MX+50 μM LaCl3, where MX is NaF, NaN3, NaCl, KNO3, KI, and NaH2PO4. In the cases of NaN3, NaCl, and KNO3, plots of the surface charge density vs. potential on the positive side of the potential of zero charge are essentially identical whether or not the multiply-charged ions are present. This indicates that there is no diffuse layer present, that the potential at the outer Helmholtz layer is zero, and that the amount of anion specifically adsorbed is equal in magnitude to the surface charge density, in accord with previous conclusions based on differential capacity measurements. In the case of 1 mM KI, the anion adsorption slightly exceeds the surface charge density at all positive values of the electronic surface charge density but may match it at lower concentrations. As fluoride is added to dilute chloride solutions no diffuse layer appears until fluoride concentrations in excess of 10 mM are reached. The observed behavior is qualitatively explained on the basis of ideas previously discussed by others5,15.