Alkyltrichlorosilanes and [2-(perfluorooctyl)ethyl]trichlorosilane (FOETS) mixed monolayers were prepared on the water subphase and were covalently immobilized onto the silicon wafer surface. Atomic force microscopic observation of the octadecyltrichlorosilane (OTS)FOETS mixed monolayer revealed that the crystalline OTS formed circular domains of ca. 1-2 mu m diameter, which were surrounded by a sealike amorphous FOETS even though molar fraction of OTS was above 75%. The alkylsilane/FOETS mixed monolayers with shorter alkyl chain length such as n-dodecyltrichlorosilane (DDTS) did not show the phase-separated structure. Also, the OTS/FOETS mixed monolayer prepared by a chemisorption method did not show distinct phase separation. Friction force microscopic observation of the OTS/FOETS mixed monolayer revealed that the OTS domain had higher frictional coefficient compared with that of the FOETS matrix. On the basis of scanning viscoelasticity microscopic observation, it was revealed that the crystalline OTS domain had higher modulus than that of the amorphous FOETS matrix. Morphological observation of the stearic acid (nonreactive)/FOETS (reactive) mixed monolayer also exhibited the phase-separated structure. The stearic acid domain in the mixed monolayer was easily extracted by solvent, and then the ghost monolayer of FOETS with holes of ca. 1-3 mu m diameter was finally obtained.