The working mechanism of bis-(3-sulfopropyl) disulfide (SPS) during Cu electroless deposition was investigated by means of a real-time observation method that measures both the open-circuit potential and the mass change of an electrode in real time. The real-time observation with the aid of a derivatization method enabled the investigation of a working mechanism of the SPS during a Cu electroless deposition. The main suggested mechanism in this study is the reductive adsorption of SPS: the SPS adsorbs onto the Cu surface in a reduced form, 3-mercapto-1-propane sulfonate (MPS), by the oxidation of formaldehyde. The MPS is then reoxidized to the SPS, reducing the cupric ion to the cuprous ion. This redox process provides an indirect route for the electron transfer from formaldehyde to cupric ions, in addition to a direct electron transfer by an autocatalytic process. When the surface concentration of MPS is low, this process can facilitate a Cu electroless deposition by accelerating the reduction of cupric ions, which is the rate-determining step of the Cu reduction. When the surface concentration is high, however, the MPS blocks the active sites for the formaldehyde oxidation rather than mediates electrons, suppressing the Cu electroless deposition. (c) 2017 The Electrochemical Society.