The fabrication of organic thin films with controlled chemical structure in the vertical direction (parallel to surface normal) is important for many practical and technological applications in organic electronics, chemical-resistant films, and biocompatible materials, among others. In order to achieve composition control in the z-direction, molecular layer deposition (MLD: covalent layer-by-layer assembly) of thin, organic films on silicon, silicon oxide, and germanium Surfaces was carried out, using the well-established UV-induced thiol-ene reaction. Through Successive contact of an interface with dithiol and diene molecules Under UV irradiation for short periods (similar to 30 min, room temperature), well-defined thin films can be obtained. Linear increases in film. thickness with respect to layer number were obtained for shorter aliphatic dienes and dithiols (C <= 8), but with longer molecules and with aromatic substrates a self-limiting Situation sets in whereby both ends of the molecule react with the surface, arresting film growth. The functionalized interfaces were characterized by ellipsometry, X-ray photoelectronic spectroscopy, and atomic force microscopy.