The direct characterization at a molecular level of the surface products formed by the interaction of aqueous solutions of ethyl and amyl xanthate with chalcopyrite polarized to different potentials were carried out by the infrared external reflection technique recently developed for detailed study of the adsorbed layer on mineral surfaces. The experimental spectroscopic data combined with the simulation of hypothetical adsorption layers have let us determine the type, structure and the surface distribution of the adsorbed species produced at different adsorption conditions, and to propose mechanisms of the interaction between the aqueous solutions of collectors and the surface of chalcopyrite. The spectroscopic results show striking differences in the composition of the outermost layer of chalcopyrite contacted with ethyl or amyl xanthate solutions, which obviously will produce differences in the flotation behavior of the mineral and in consequence will influence its separation from other ore components. These observations explain why different xanthate homologues should be used in flotation practice. A detailed discussion of flotation pulp solution conditions to produce different types of hydrophobic surface species for either collector and collectorless flotation of chalcopyrite is also presented.