Cu electrodeposition onto Ru during dual damascene processing is complicated by the possible presence of Ru oxide, which is known to exist at more cathodic potentials than predicted by its Pourbaix diagram. Several research groups have proposed cathodic treatments prior to Cu electrodeposition to reduce Ru oxide, but these methods were developed semiempirically. We report voltammetry and impedance studies of Ru oxide reduction in 1.25 M H2SO4. Voltammetry studies suggest that Ru oxide reduction may be difficult for samples exposed to laboratory air for extended times. Impedance studies show a clear signature of Ru oxide reduction at a potential of about -300 mV vs saturated calomel electrode, as indicated by both a dramatic reduction in the charge transfer resistance and the appearance of a low frequency inductive loop. The time response of the impedance at 5 Hz following a potential step to several potentials cathodic to -300 mV indicates that Ru oxide reduction occurs quite rapidly. (C) 2010 The Electrochemical Society. [DOI: 10.1149/1.3519848] All rights reserved.