The degradation of bis(3-sulfopropyl) disulfide (SPS) during Cu electrodeposition and the influence of its decomposition products on the Cu film properties are investigated. SPS decomposes into 3-mercapto-1-propane sulfonic acid (MPSA) and 1,3-propane disulfonic acid (PDS) by dissociation of the disulfide bond and oxidation reactions. The MPSA seems to recombine to form SPS through a reaction with cupric ions, whereas the PDS accumulates in the plating solution without any further reactions. It was found that the net conversion from SPS to PDS reduced the acceleration effect and resulted in the deterioration of Cu feature filling. The final decomposition product, PDS, caused no remarkable changes in either the electrochemical behavior or the film properties. The deterioration in feature filling by degradation of SPS is mainly associated with the decrease in SPS concentration by decomposition and incorporation. (C) 2013 The Electrochemical Society. All rights reserved.