Abstract: The a.dsorption of thiourea . (TU) and diphenythiourea (DPTU) on mercury has been investigated by interfacial tension measurements in aqueous solution and aqueous methanol. The maximum potential of electro-capillary curve shifts toward negative as TU or DPTU concentration increases. It is assumed that the negative parts (S atom) of the molecules facing the mercury surface are adsorbed resulting in a linear relation between Δφ_s=0 and lgC. If the mercury stirface charge density equals to ε, the adsorption isotherm of TU. can be calculated by the follwing equation. Γ=-1.44×10^-10lgC + (4.45 + 0.19ε) × 10^-10