Ultrasmooth octadecyltrichlorosilane (OTS) monolayers (2.6 +/-0.2 nm thick, RMS roughness similar to1.0 Angstrom) can be obtained reproducibly by exposing clean native SiO2 surfaces to a dry solution of OTS in Isopar-G. A clean room is not required. Atomic force microscopy (AFM), X-ray photoelectron spectroscopy (XPS), contact angle data, and ellipsometry show that film formation occurs through a "patch expansion" process and terminates once a single monolayer is formed, after about 2 days. These monolayers are suitable as substrates for high-resolution electron beam and AFM or STM lithography. Further observations highlight the importance of controlling water content during deposition of siloxane self-assembled monolayers. OTS covers the surface much faster when there is a little water in the OTS solution; contact angle and ellipsometry data indicate formation of a hydrophobic, 2.6 mn thick film after about 2 h. However, these OTS films have a totally different growth mechanism than films grown from dry solutions and are not really monolayers. The OTS forms platelike islands that then adsorb onto the surface; the resulting overlayers have RMS roughness of more than 3 Angstrom. Continued exposure to the OTS solution results in continued island deposition and increased roughness.