A description is given of the mixed electrochemical potential measured in situ during the chemical-mechanical polishing of copper. Potential measurements are indicative of the dissolution rate of copper and of the equilibrium form of the polished copper by-products. These measurements are used to explain the polish performance in several ammonia-based slurries. Specifically, the polish rate is shown to correlate with the potential and the change in potential during polishing. In addition, complexing of copper ions with dissolved ammonia is discussed and shown to be ah effective method for increasing the solubility of copper ions in the slurry and thereby increasing the polish rate.