Bath stability monitoring of electroless copper deposition has been done with different methods. A combination of UV-VIS spectra and pH measurement enabled us to determine the degradation of an electroless Cu solution. The pH of the Cu-containing solution gradually decreased as a function of time, while the UV-VIS absorbance increased. The decrease of pH is due to the consumption of OH- by the deposition reaction and the Cannizzaro reaction of glyoxilic acid. As application of these methods, the effect of the preparation sequence and bath stability was evaluated, bath break down was detected by the different methods used, and the potential for process monitoring was demonstrated. Furthermore, pH control during deposition was found to be very important to avoid defects in the electroless deposited seed layer, close to the via opening and top surface. (C) 2012 The Electrochemical Society. [DOI: 10.1149/2.070207jes] All rights reserved.