We investigate the influence of chloride Cl- ions in a broad range of chloride concentrations on the kinetics and mechanism of copper electrodeposition from sulfate-based acidic electrolytes. Chloride ions influence copper deposition through two competitive effects: at low Cl-concentration (few mM), chloride ions depolarize the Cu reduction process, while higher Cl- concentrations induce complexation of copper species and cause a cathodic polarization of the deposition process. Cu reduction proceeds through two parallel mechanisms, a direct two-step reduction and a chloride-mediated route, whose relative importance depends on the amount of chloride present. A transition between these two mechanisms can be identified both by steady-state and impedance methods; however, the chloride concentration at which it occurs depends on the time scale probed by the two techniques. Impedance measurements further demonstrate that the presence of chlorides changes the double-layer structure. (c) 2007 The Electrochemical Society.