The ferric/ferrous redox couple interacts with bis-(3-sulfopropyl) disulfide (SPS), leading to increased amounts of 3-mercaptopropyl sulfonate (MPS) in solution. SPS is an essential organic additive in the electrochemical deposition of copper from acidic cupric sulfate electrolytes onto printed circuit boards and integrated circuits. Electrochemical studies show that the accelerating action of SPS is dependent on the ferric/ferrous concentration ratio, with weakening dependence at increasing cupric sulfate concentration. HPLC and electrochemical results suggest that an increased concentration of MPS in the presence of the redox couple is the primary cause of the increased acceleration. Equilibrium estimates of MPS generation by the introduced redox chemistry partially explain these observations; the standard reduction potential of SPS to MPS reduction is estimated to be in a range between 0.3 and 0.4 V vs. SHE. However, the amount of acceleration goes through a maximum at an intermediate ferric-ferrous ratio, possibly because SPS may be converted to MPS via both an oxidative and a reductive pathway. (C) 2011 The Electrochemical Society. [DOI: 10.1149/1.3593018] All rights reserved.