The adsorption of glutaric acid on mercury in aqueous solution of 0.1M Na2SO4 has been investigated by capillary electrometer. Thermodynamic parameters such as surface charge density (qM), surface excess of organic molecule adsorbed (Гorg) and coverage (θ) have been evaluated. The adsorption of glutaric acid occurs both on positively and negatively charged mercury surface. Maximum adsorption occurs at negative charges and desorption occurs at extremes of positive and negative charges. It is found from qM versus potential (‒E) plot that the glutaric acid at various concentrations intersects one another at 950 mV which is negative to the electrocapillary maximum (ecm) and at a charge density of ‒2.5 μC cm‒2. The adsorption of glutaric acid obeys Langmuir's adsorption isotherm. The maximum coverage occurs at a charge density of ‒1.2 μC cm‒2. The free energy changes of adsorption of glutaric acid with coverage were ascribed to the dipole-dipole interaction at the surface.