The preparation, structure, and mechanical stability of self-assembled monolayers formed by octadecyltriethoxysilane (OTE) on mica have been studied by atomic force microscopy. The nanometer scale morphology of the films (3-D clusters, pinholes, etc.) is compared for various preparation methods and correlated with their macroscopic wettability. High-resolution images of the atomically smooth monolayer (1.5 Angstrom root mean square roughness) reveal the existence of only short range order. By applying a load above 10 nN with sharp Si3N4 tips (radius < 300 Angstrom), the film could be removed leaving 25 Angstrom deep holes. Using d the same tip, thiol monolayers on gold could be displaced at loads approximate to 5 nN, although, in this case, the displacement was reversible. In contrast to the case of OTE, films formed by octadecyldimethylmethoxysilane (ODMS) showed the presence of only 3-D clusters with poor adhesion to the mica substrate. On the basis of these results, we conclude that the mechanical strength of the films formed by OTE is due to siloxane cross-linking between molecules rather than to chemical bonding to the mica substrate.