We devise a novel electrochemical method that enables the analysis of the average molecular weight (MW) of polyethylene glycol (PEG) in an acidic Cu plating bath. During the cyclic voltammetry analysis of the Cu plating bath, a hysteretic voltammogram is generated, and it is related to the MW of PEG. This relationship is used for the analysis of the average MW of PEG in the acidic Cu plating solution, with the error of analysis being less than 9%. (C) 2016 The Electrochemical Society. All rights reserved.