An in situ optical probe of ultrathin silicon oxide dissolution has been developed based on changes in the concentration of chelated Cu2+ neat the surface of a silicon wafer during diffusion-limited deposition of Cu. A laser is reflected from the surface of a silicon wafer at glancing incidence, which allows the diffusion layer to be sampled over about 7% of the laser path, while the laser intensity is monitored for changes in the optical absorption of chelated Cu2+. This method has been employed to measure the average dissolution rate of ultrathin silicon oxides into dilute HF solutions of various strengths. The silicon oxide layers were approximately 8.6 Angstrom thick, as measured by spectroscopic ellipsometry, and were formed by aqueous cleaning. The dissolution rates were on average 24% lower than those measured previously for thick thermally grown oxides into HF solutions. The reduction in the dissolution rate is consistent with the known densification of silicon oxide near the Si/SiO2 interface.