The mechanism of copper contamination of silicon wafers from dilute HF solutions containing ultratrace levels of metallic ion impurities, was investigated using a new electrochemical cell, which proved to act as a very efficient sensor for in situ characterization. Upon copper contamination, the open-circuit potential was observed to shift rapidly toward more positive values at a rate nearly proportional to the copper concentration. All potential/time curves tend to reach a plateau, while quantitative measurements using radioactive tracers revealed that during a few tens of minutes, copper ions were continuously reduced on the silicon surface. Results are interpreted in terms of the mixed-potential theory and lead to the conclusion that copper nuclei act as a catalyst which enhances the cathodic activity for proton reduction. The model was supported by atomic force microscopy observations which showed the initiation of corrosion pits around the nuclei.