The ability of various quaternary ammonium cations (QACs) to inhibit Cu electrodeposition was investigated qualitatively and quantitatively via cyclic voltammetry. With addition of the dodecyltrimethylammonium cation (DTA(+)) as a representative QAC in the electrolyte for Cu electrodeposition, we observed inhibition of electrochemical Cu2+ reduction through surface adsorption, regardless of the electrode material used (Cu, Ag, Au, glassy carbon, and fluorine-doped SnO2). By examining various QACs with different structures, it was determined that the hydrophobic tail of DTA(+) allows it to act as an inhibitor of Cu electrodeposition. DTA(+) interacts strongly with anions (sulfate, chloride), which causes the hysteresis observed in cyclic voltammograms on Cu rotating disk electrode and indicates that its inhibition function is associated with the formation of the surface aggregates of DTA(+) on anion-adsorbed Cu surface. Adsorbed DTA(+) reduces the surface roughness of Cu electrodeposits but is not significantly incorporated in the deposit, as confirmed by SEM, AFM, and XPS analyses. Graphic