The effect of triethanolamine (TEA) as a second ligand was studied on the electrodeposition of copper in 1-hydroxyethylene-1,1-diphosphonic acid (HEDPA) electrolyte. The electrochemical behavior of copper deposition was investigated by cyclic voltammetry and electrochemical impedance spectroscopy. The introduction of TEA suppresses the electrodeposition of the copper and promotes the dissolution of copper anode, resulting in fine grains and smooth bright deposits. The "Coordination-Adsorption" theory was proposed to explain the mechanism of action of TEA. The introduction of TEA in the HEDPA electrolyte induces the formation of the CuTEA(OH)(2) coordination compounds. The formed CuTEA(OH)(2) coordination compounds adsorb on the electrode surface and inhibit the growth of the copper crystal nucleus. (C) The Author(s) 2017. Published by ECS. All rights reserved.