High-quality alkyl monolayers on silicon have been prepared from 1-alkenes and hydrogen-terminated Si(111). The 1-alkenes form monolayers upon free-radical initiation with diacylperoxides. Heat also initiates monolayer formation, although monolayers prepared from heated long-chain l-alkenes are of lower quality than those prepared with free-radical initiation. Even when a high concentration of diacyl peroxide is used to initiate monolayer formation, the l-alkene is the primary constituent of the monolayer. Alkynes also form monolayers on silicon when initiated by diacyl-peroxides, X-ray reflectivity shows that the monolayer thickness is of molecular dimensions and that the density is close to that of crystalline hydrocarbons (similar to 90%). Infrared spectroscopy shows that the alkyl chains in the monolayers are densely packed. Infrared dichroism shows that the chains are tilted from the surface normal and twisted about their axes. The wetting properties of the monolayers show that they are methyl terminated. After many weeks of air exposure, the silicon substrate under the monolayers is not significantly oxidized. The monolayers are very stable to boiling chloroform, boiling water, boiling acidic and basic solutions, and fluoride and are at least as stable as similar chain-length monolayers prepared from trichlorosilanes on oxidized silicon. We propose that alkyl chains in the monolayers are bound to the silicon substrate through silicon-carbon bonds and compare a proposed mechanism of bond formation to analogous homogeneous reactions.