In situ infrared reflection spectroscopy combined with spectral simulation was applied to the study of the supramolecular structure of ethyl xanthate (C2H5OCS2-) species on cuprous sulfide. The surface layer was produced electrochemically from aqueous solution. Two adsorption products are observed at potentials above 350 mV (SHE) : the cuprous ethyl xanthate complex (C2H5OCS2Cu) and liquidlike form of dixanthogen (C2H5OCS2)(2). The detailed compositional and structural characterization is made on the basis of the comparison of simulated and experimental spectroscopic results. The quantitative evaluation of the two-component heterogeneous layer is discussed in detail. The in situ cell configuration used in this study was tailored specifically for the investigated system.