The oxidation and reduction of copper (Cu), either electrodeposited or vapor deposited onto quartz crystal resonators, was examined in borate/boric acid mixtures at two different pH values, comparing electrochemical measurements with mass changes. The results showed that at pH 8.8, Cu dissolution significantly influences the microbalance readings. It was found that, when surface oxidation during potentiodynamic scans is limited in the anodic direction, the first reduction peak is caused by Cu redeposition and not by oxide reduction. At pH 10.0, Cu dissolution is minimal, and the microbalance signal is given by surface oxidation or reduction. Comparing charge and mass data, it was found that the reaction Cu+ <----> Cu is the formation or reduction of Cu2O, while the reaction Cu2+ <----> Cu+ most likely involves some hydroxide. (C) 2004 Elsevier Ltd. All rights reserved.