Microelectronics is a new field of interest for electrochemistry as this technology has adopted copper electrodeposition to build the interconnections in microcircuits by means of the so-called Damascene process. In this work, Electrochemical Impedance Spectroscopy (EIS) was used to investigate two critical stages of this process, namely copper deposition itself in sulfuric-sulfate medium in the presence of organic additives and post-chemical-mechanical polishing (CMP) cleaning in oxalic acid medium. Models were proposed for the two systems and they were tested under various experimental conditions. It was shown in particular, that EIS could be used to monitor the ageing of the copper plating bath.