The adsorption of the macrocyclic ligand kryptofix 222 at the mercury-water interface has been studied as a function of the polarization potential and the ligand concentration. An automated apparatus has been used to measure interfacial tension by the drop-shape technique. The relative surface excess and the surface charge density have been obtained, and from these parameters, the molecular packing at the interface and the free energy of adsorption have been calculated. The results show that the macrocyclic ligand strongly adsorbs at the interface in the presence of an intermolecular repulsive electrostatic contribution due to Na+ cations trapped inside the macrocyclic cage. Moreover the adsorption driving force is similar to that reported for the adsorption of aliphatic compounds, i.e., the hydrophobic repulsion of the exposed molecular surfaces.