The mechanism of copper deposition in a sulphuric acid–sulfate bath in the presence of chlorides was investigated by electrochemical impedance measurements. A model where the chloride ions added to a sulphuric acid–sulfate copper-plating bath have a catalytic action not limited by mass transport has been proposed. This shows that the observed mass transport limitation is due to the diffusion of cupric ions and that, at high current densities, the copper deposition is mainly due to a mechanism involving CuCl formation on the electrode.