An optical technique based on simple absorption spectroscopy has been demonstrated for monitoring metal contaminant deposition from aqueous processing solutions widely employed during microelectronics fabrication. Cu deposition from 0.15 and 0.25% HF solutions contaminated with 3.5 ppm Cu has been observed as a decrease in the absorption of a HeNe laser reflected at glancing incidence from a Si wafer. This is caused by the depletion of Cu2+ from the mass-transfer boundary layer, providing direct evidence that Cu deposition is rate-limited by diffusion. This technique allows determination of which metallic species can deposit onto Si wafers in diffusion-limited processes during various aqueous processing steps in microelectronics manufacture. In addition, Cu deposition is seen to coincide with the completion of SiO2 dissolution, confirming that Cu2+ reduced by an electroless process involving simultaneous Si oxidation. The decrease in absorption is consistent with a mass-transfer boundary layer thickness of about 335(20) mu m. The optical absorption returns to its original value about 2 min after the onset of deposition, consistent with diffusion-limited deposition of one monolayer of Cu, which is followed by a transition to a slower, surface rate-limited process.