The spontaneous adsorption of O-isobutyl-N-(ethoxycarbonyl)thionocarbamate (C4H9OSCNHCOOC2H5, TC) on a semiconductor (Cu2S) has been studied by infrared external reflection spectroscopy (ERS) and X-ray photoelectron spectroscopy (XPS) in order to elucidate the molecular orientation and coordination of the thionocarbamate-metal complex in the adsorption layer. It has been observed that since cuprous sulfide is a low-absorption material in the infrared region, a set of surface selection rules different from those applicable to metal substrates has to be used. Both positive and negative absorption bands were observed in the recorded spectra of an adsorption layer, depending on the angle of incidence and the polarization of the incident radiation. The intensity of an infrared absorption band in the reflected radiation was calculated for a three-phase adsorption system by using assumed optical constants for the adsorption layer, at various angles of incidence. Both p- and s-polarizations have been considered, and calculations have been made for all three electric vector components at the interface. Generally good agreement was obtained between experimental and theoretically calculated absorbance values for the system investigated. It has been shown that the infrared reflection method can be used to determine both the chemical nature and the orientation of individual molecular groups of adsorbed species on low-absorption material at mono- and multilayer coverages. Application of the reflection method, supported by theoretical calculations, for determining the thickness and optical properties of the adsorption layer will also be discussed. Both the infrared and XPS results showed that the thionocarbamate molecules are oriented in the adsorption layer with their aliphatic chains toward the solution at monolayer coverage. The six-membered complex Cu(TC') (TC' = C4H9OSCN-OOC2H5), with dissociation of the N-H bonding in the thionocarbamate molecule, forms the adsorption layer. As the surface coverage increases, the position of the adsorbed molecules changes gradually, and they become randomly oriented.