The specific adsorption of penicillin G (salt K) on the mercury in aqueous solution has been determined from electrocapillary and capacity measurements. The adsorption of penicillinium anions can be fitted by a Frumkin isotherm with a value of -3.5 for the interaction parameter. The standard Gibbs energy of adsorption, Delta G(0), is a linear function of the surface charge and the Delta G(0) value at zero charge is -38.6 kJ mol(-1). The inner layer capacity is analysed into its components and these are related to the distances from the inner and outer Helmholtz planes to the metal surface. The possible orientation of the antibiotic on the electrode surface is discussed.