Cupric ions are a major component in electrolyte baths for Damascene copper plating. To unravel the mechanism of this plating process, knowing the cupric ion activity (a(Cu2+)) is essential. However, determining a(Cu2+) in such solutions is not straightforward. Calculating the activity often requires complicated models to account for the complex composition and high concentration of the electrolytes. Also experimentally obtained data for these particular solutions are hardly available. In this work, the cupric ion activity in the presence of sulfuric acid is determined by measurements of the open-circuit Cu electrode potential in these solutions. A saturated mercury/mercurous sulfate electrode (SMSE) is used as the reference electrode. Because the potential of this electrode appeared to depend on the solution composition, a silver/silver chloride electrode was used for calibration of the SMSE and for determination of the precise copper electrode potential vs. the standard hydrogen electrode. Consequently, it was possible to calculate a(Cu2+) in highly concentrated multi-component solutions. (C) 2012 The Electrochemical Society. [DOI: 10.1149/2.050302jes] All rights reserved.