A technique has been developed for the study of the reaction mechanism and the monitoring of copper electroplating baths utilizing a combination of solid phase extraction and high-performance liquid chromatography. This method appears suitable for live electroplating bath control as well as for organic identification, concentration measurement, and reaction mechanism studies. The results from the experiments carried out with the technique described in this paper show how the monitoring of a few diagnostic peaks recovered from a live bath sample can be directly used for this purpose. Two copper sulfuric plating baths at different acidity were used for this study. The stability, degradation mechanism, and by-products of bis(sodiumsulfopropyl) disulfide were addressed in Part I, while polyethylene glycol (PEG) behaviors are studied in Part II. The identification of PEG breakdown by-products was achieved in this work. (c) 2005 The Electrochemical Society.