This paper documents an experimental investigation of additive behavior in acidic copper plating solutions containing chloride ions, polyethylene glycol (PEG), and bis-(3-sodiumsulfopropyl disulfide) (SPS). These solutions represent a simplified model of solutions used industrially for electroplating copper interconnects. Experiments were conducted with use of a rotating disk electrode under both galvanostatic and potentiostatic conditions and the transient behavior was observed. Linear sweep voltammetry was also performed. A transition time for the system to reach steady state was observed under both galvanostatic and potentiostatic conditions and found to be a strong function of SPS concentration. Superfilling is made possible by the existence of a long transition time that permits establishment of a significant rate difference between the top and bottom of the cavity. Experimental results provide evidence for slow adsorption and desorption of the accelerator and SPS incorporation into the deposit. Linear sweep voltammetry indicates that behavior of the accelerator is potential dependent. The same mechanism used to explain superfilling can be applied to explain the leveling/brightening of copper surfaces in these solutions. (C) 2003 The Electrochemical Society.