A new model for the diffuse double layer which accounts for the effects of ion size and solution permittivity is described. It is then used to estimate the potential drop across the diffuse layer at negative charge densities for the cases that Li+ and Cs+ are the electrolyte cations. The potential drop in the Li+ system is considerably smaller than that in the Cs+ system at 1 M, and both values are smaller than the value predicted by the Gouy-Chapman model. As the electrolyte concentration decreases these differences become smaller so that at 0.01 M, the present model predicts that the diffuse layer potential drop is approximately 90% of the Gouy-Chapman estimate. The results of the model are used to examine the differences in inner layer structure at mercury electrodes with Li+ and Cs+ ions at the outer Helmholtz plane, and to reconsider the question of the specific adsorption of Cs+ at negative-charge densities.