Partial octadecylsiloxane monolayers were formed on borosilicate glass substrates and characterized using friction force microscopy (FFM) and contact angle techniques. Use of the friction force microscope allowed the film composition to be investigated despite significant roughness of the underlying substrate. The height variations of the substrate were of the same order as the film thickness : a situation in which conventional atomic force microscopy can produce ambiguous or inconclusive results. Substrates with a range of partial surface coverages were prepared, and contact angle measurements demonstrated the expected decrease in wettability with increasing siloxane coverage. Friction force microscopy revealed a corresponding decrease in the friction coefficient of the surface with increasing coverage. Microscope resolution determined an upper limit of 15 nm for the lateral size of isolated siloxane domains for all coverages.