Cationic surface excesses of Li+, Na+, Mg2+, and Al3+ are obtained from electrocapillary curves corresponding to aqueous perchlorate solutions. They are compared with earlier reported data on chlorides, obtained under similar conditions. The influence of ionic sizes on the ion-free layer thickness is discussed in the framework of Gouy-Chapman and the generalized UDCA theory. Differences between the; surface excesses of a given cation in the presence of perchlorate and chloride find a plausible explanation in terms of the anion distance of closest approach to the electrode.