Soft thin films, i.e., organic monolayers, are assembled on Si(100) from the chemical vapor deposition (CVD) of amine molecules (R-NH2) on the monochloride-covered surface. The N anchor is believed to be bridged between two surface Si atoms while the hydrocarbon group remains intact. Thermal stability of the resulting organic monolayer depends on molecular structure. In the phenyl assembly, which forms close packing on the Si(100) surface, the molecular monolayer is thermally stable; desorption and dissociation only occur at temperatures above 750 K. Alkyl assemblies are less stable; thermal desorption of hydrocarbons occurs below 700 K. The method demonstrated here is sufficiently general and can be extended to other CVD reactions for the covalent assembly on silicon surfaces in either vacuum environment or solution phases.