Cu electrodeposition on thin-film, highly resistive, Pt electrodes was studied experimentally. Electroplating additives can have a large impact on thickness uniformity across long resistive electrodes. Results are compared to simulations, and it is shown that nucleation influences thin-film uniformity and that electroplating additives may also cause an increase in the nonuniformity due to the spatial variation in their acceleration or inhibition across the electrode. The spatial variations in suppression are particularly strong in the presence of bis (3-sulfopropyl disulfide), whose deleterious effect can be reduced by a leveler. (c) 2008 The Electrochemical Society.