Surface modification of metal surfaces with inorganic clusters is very important for electrocatalysis. We present here the growth of an osmium carbonyl cluster on a self-assembled alkylsiloxanethiol monolayer on a Au surface. This has been done by the surface reaction of Os-3(CO)(11)(NCCH3) (1) with a self-assembled monolayer of (3-mercaptopropyl)trimethoxysilane (MPS) on Au(111). The determination of the surface morphology and chemical composition has been done by using scanning tunneling microscopy (STM) and X-ray photoelectron spectroscopy (XPS). The reaction of the osmium cluster with the alkylsiloxanethiol monolayer destroys the well-ordered thiol surface by forming several cluster aggregates which range from 10 to 22 Angstrom in diameter.