We present a study of the effect of chloride anion additions on the electrochemical nucleation and growth of copper on n-Si(100) from acidic sulfate solutions. Chloride additions in the millimolar range depolarize copper reduction, while additions in the 100 mM range and above have the opposite effect. Chlorides have no effect on the nucleation and growth mode. Deposition at constant potential in the presence of small amounts of chloride leads to an increased deposition overpotential, but contrary to the predictions of nucleation theory, this results in a lower density of nuclei with larger size. We hypothesize that adsorption of chlorides on the copper islands enhances surface diffusion processes which lead to island coarsening through Ostwald ripening. Morphological changes during deposition are possible at room temperature due to the high values of diffusivities at the electrolyte/solid interface. (C) 2007 The Electrochemical Society.