The adsorption of several types of alkylamine molecules on individual semiconducting and metallic single walled carbon nanotubes is investigated by electrical transport measurements. The electrical conductance of semiconducting nanotubes changes drastically upon adsorption of butylamine and 3 '-(aminopropyl)triethoxysilane in the vapor phase. The amine groups in the molecules are electron donating and responsible for charge transfer to the semiconducting nanotubes, and thus, the observed electrical conductance change. Molecular adsorption on metallic single-walled nanotubes causes no significant change in the electrical conductance of the nanotube samples, consistent with earlier finding that the electrical properties of metallic nanotubes are relatively insensitive to molecular adsorption. Adsorption of the amine compounds on partial lengths of semiconducting nanotubes causes modulated chemical gating and intramolecular wire junctions that exhibits pronounced rectifying diode behavior. The physisorbed;amine molecules desorb from nanotubes typically in 12 h, restoring the nonrectifying characteristics of the samples. Cross-linking of the heterofunctional 3 '-(aminopropyl)triethoxysilane molecules on partially exposed nanotubes via siloxane bridges leads to permanent intrananotube rectifiers.