Organic additives play an important role in regulating film growth and properties in Cu electrodepositlon. 3-Mercapto-1-propane sulfonate (MPS) and its dimer bis(3-sulfopropyl) disulfide (SPS) are representative accelerators that promote Cu electrodeposition rate. In this study, the formal reduction potential of SPS to MPS was determined by measuring the equilibrium constants of thiol-disulfide interchange reactions with the reference thiol/disulfide pairs by H-1 nuclear magnetic resonance (H-1 NMR) spectroscopy. The calculated formal reduction potential of SPS to MPS was 0.153 V (vs. standard hydrogen electrode). Based on this calculation, the cell potentials of Cu(I)(thiolate) formation reaction with either SPS or MPS, which is a key reaction in accelerating mechanism, were also estimated by ultraviolet-visible absorption and H-1 NMR spectroscopies: they were found to be 0.202 V and 0.214 V for SPS and MPS, respectively. The positive cell potential indicates that the formation of Cu(I)(thiolate) complexes is thermodynamically favorable in the presence of additives.