This study describes an optical reflectometry setup capable of detecting adsorption at a liquid/solid interface. This setup has improved sensitivity over the previous reflectometry instruments discussed in the literature. This improvement is achieved by implementing a stabilized He–Ne laser and lock-in detection scheme. The high sensitivity of the present setup was demonstrated by probing the formation of the electrical double layer at the water–silica interface. Quantitative interpretation of the data was achieved with the basic Stern model. The determined value of the single-ion refractive index increment for Na+ was 7.1 ± 0.2 mL/mol. The detection limit of the present setup is <1 μg/m2, which is comparable to modern surface plasmon resonance instruments, and is 10 times better than currently achieved with quartz crystal microbalances and other optical surface sensitive techniques.