The effects of suppressor and accelerator on the superfilling of copper are studied for two commercial chemistries. The potential transients during galvanostatic plating were obtained for injections of the industrially recommended dose of accelerator and various doses of suppressor. The potential increase immediately after the injection was found to be strongly dependent on the type of suppressor as well as the amount of suppressor injected. While the transient of the full dose of suppressor represents what happens at the mouth of the feature and the field outside the feature, the bottom of the feature can be simulated by the case in which a fraction of suppressor is injected. The filling results in sub-100 nm lines are well correlated with the differences observed in the potential transients.