The results of a study of double layer capacity at the mercury/acetonitrile interface in the presence of alkali metal and tetraalkylammonium perchlorates are presented. It is shown that the Li+, Na+, tetraethylammonium, and perchlorate ions are not significantly adsorbed over most of the polarizable range of the electrode, whereas the K+ and Cs+ ions are adsorbed at more cathodic potentials. Inner layer capacity data are presented and discussed in terms of the specific effects of ions on interfacial structure.