Si(111) surfaces with covalently attached monolayers have been investigated by optical second harmonic generation (SHG). The surfaces are modified by wet chemical procedures and studied by SHG in air or in a liquid immersion cell, using fundamental wavelengths lambda = 775 and 830 nm. Results are reported for monolayers including -H, -C10H21 (decyl), -O-C10H21 (decyloxy) and -Cl, and for a Si(111) surface with a native oxide film. The rotational anisotropy of the SHG efficiency has been studied and estimates have been made of the relative values of nonlinear susceptibilities for the surface and bulk response. The mechanisms that contribute td SHG are identified. The application of SHG for in situ monitoring of chemical reactions of Si(111)-H is demonstrated, and results are presented for the formation of Si(111)-Cl in a UV photoinduced reaction in liquid CCl4.