Previously, a model of copper deposition in the damascene process was proposed that takes into account the existence of a copper-accelerator complex as well as the competitive adsorption of an inhibitor and an accelerator. The model was verified experimentally by electrochemical impedance spectroscopy and dc voltammetry measurements on fresh solutions of a superfilling, copper deposition plating bath. In this paper, the same model was applied to plating baths during copper deposition where aging and possible relaxation of the bath back to a steady state can occur. The model can provide insight into interpreting the impedance spectra during and after copper deposition. Changes in the Cu-accelerator complex in the bulk solution and the surface coverage of this complex were followed using a parameter based on the low-frequency inductive loop of the electrochemical impedance spectra. Results indicate that the impedance spectroscopy measurements are mainly sensitive to Cu-accelerator complex formation and destruction.