Treatment of silicon or gallium arsenide with the refluxing vapors of 3-aminopropyltriethoxysilane in toluene or xylene solution results in a strictly monolayer coverage of the surface with aminopropylsilyl groups. Electron microscopy indicates that these conditions avoid the formation of polymeric globules whose presence is characteristic when the substrate is immersed in the treating solution. Ellipsometric measurement of the thickness change upon reversible adsorption of dodecyl sulfate anions indicates a closely packed layer of extended hydrocarbon chains. This implies that the coverage of the semiconductor surface with the underlying aminopropyl groups is essentially complete. Neither the rate of the silane attachment reaction nor the rate of hydrolysis is strongly affected by the doping level of the semiconductor. Reactions of the amino groups allow bonding of a variety of organic molecules to the surface. Particularly facile are (a) carbodiimide coupling of carboxylic acids, (b) conversion of the surface to a strongly acidic one, and (c) conversion of surface amino groups to isothiocyanate followed by coupling to primary amines. © 1978, American Chemical Society. All rights reserved.