The adsorption of C.I. direct red 73, a bisazo direct dye at the mercury/solution interface from 1 M solutions of NaCl, NaNOJ and NazS04 has been investigated using capillary electrometer. Thermodynamic parameters like charge on the metal surface (qM), the surface excess of organk molecule adsorbed (foil)' surface coverage (8) and free energy of adsorption (-AG) are evaluated and presented. The adsorption of C.I. direct red 73 obeys Langmuir's adsorption isotherm in all the three base electrolytes