The differential capacitance of a polycrystalline Ag electrode was measured in a NaClO4 electrolyte containing Na2S2O3 at concentrations ranging from 0.1 to 20 mM and at electrode potentials ranging from -0.9 to -0.3 V vs SCE. The differential capacitance measurements were analysed to obtain the surface coverage of specifically adsorbed thiosulfate (S2O32-) as a function of both electrode potential and bulk concentration. The various forms for the adsorption isotherm at an electrochemical interface which are commonly employed are reviewed and discussed. The adsorption behaviour is best explained by the formation of image dipoles by the specifically adsorbed ions, whose energetic interactions are then dominated by repulsion of like dipoles. The adsorption data is quantitatively fit to an adsorption isotherm for interacting dipoles, yielding an effective dipole moment of 0.72 D for the system S2OS32-/Ag.