Adsorption phenomena at the mercury electrode/cucurbituryl aqueous solutions are studied by the measuring of the electrode differential capacitance C as a function of potential E. The data obtained showed that the adsorption potential region is abnormally wide (>2 V). Two segments are observed in the C,E-dependences, which relate to adsorption layers with different structure. The complicated adsorption layers forming in the studied systems can be explained by the structure of the cucurbituryl cavitand whose complexes with inorganic cations are formed by the cation binding by oxygen-containing groups of external portals, rather than their inclusion into the cavitand’s inner cavity as in the case of cryptate formation. Adsorption parameters for adsorbate layers formed in the cucurbituryl + Na2SO4 solution are calculated. The data obtained evidence a rather strong potential dependence of the properties of the adsorption layers formed at the electrode/solution interface in the studied system.