Alkyl and fluorinated alkyl monolayers covalently bonded to the silicon (111) surface have been prepared by UV illumination of the H-Si(111) surface while immersed in a solution of olefin precursor under high vacuum. An alkoxy monolayer covalently bonded to the silicon (111) surface is formed by the reaction of the H-Si(111) surface with a heated solution of primary alcohol precursors under high vacuum. Ellipsometry, X-ray photoelectron spectroscopy, and atomic force microscopy are used to characterize the three monolayers. The alkyl monolayer is measured to have the largest number of organic adsorbates per surface silicon atom. Properties of the monolayers on the silicon surface are probed by cyclic voltammetry. The hydrophobic alkyl monolayer slows the oxidation of the silicon surface by water. The three monolayers also slow the rate of electron transfer across the silicon-electrolyte interface by acting as a tunneling barrier. The alkyl monolayer in tetrahydrofuran exhibits a large and reproducible blocking of the electron transfer.