Optical second-harmonic generation (SHG) is used to probe Si(111) surfaces with covalently attached monolayers. The surfaces are prepared using wet chemical modification to form -H, -C 10 H 21 (decyl) and -O-C 10 H 21 (decyloxy) monolayers. The SHG efficiency for these surfaces as well as a Si(111) surface with a native oxide film is reported. The signal's intensity and modulation through a 360 ̕rotation, or rotational anisotropy, can be correlated with the substrate's chemical modification. The potential for SHG to be used as a probe for monitoring the initial oxidation of Si(111)-H in situ is investigated. The mechanism by which this reaction occurs over time is not well understood. Three environments have been studied: N 2 purge to inhibit oxidation, oxidation in ambient air, and photo-oxidation in ambient air using UV irradiation. Possible processes by which water and molecular oxygen initiate the surface oxidation are identified.