Electrochemical impedance spectroscopy and cyclic voltammetry were used to characterize the aging of copper plating baths used in damascene process for superfilling of trenches and vias in the semiconductor industry. The effects of copper anode composition, anodic current density, and accelerator nature and concentration were studied. Phosphorus added to copper anodes has a prominent impact on the evolution of current/voltage curves and impedance spectra during bath aging. The observed evolutions were explained by the formation of a -accelerator complex in the bulk of the bath that adsorbs at the copper surface to act as an accelerating species, and by the increase of its concentration during aging with undoped copper anode. The -accelerator complex formation is limited by the phosphorus contained in the Cu-P anode, which traps electrogenerated cuprous ions in the black anodic film. Differences observed between baths containing SPS (bis(3-sulfopropyl)disulfide) or MPSA[(3-mercaptopropane)sulfonic acid] are also attributed to the existence of two different -accelerator complexes, both of which exhibit different formation equilibrium constants.