The flotation behavior of minerals depends on the balance between the hydrophobic and hydrophilic species present at the surface of the minerals. The direct characterization of the interface products formed by the interaction of an aqueous solution of ethyl xanthate (C2H5OCS2-, X(-)) with chalcopyrite (CuFeS2) and with mineral samples containing mainly tetrahedrite (Cu12Sb4S13) and tennantite (Cu12As4S13) was carried out by infrared Fourier transform (FT) reflection spectroscopy. The experimental data collected from in situ attenuated total reflection (ATR) and ex situ diffuse reflectance (DRIFT) measurements have allowed us to determine different types of adsorption products and to propose mechanisms of the interaction between an aqueous solution of surfactant and the surfaces of the investigated minerals. It is shown that in situ studies provide more complete information about the composition of the adsorbed layers. It is revealed that two major processes take place at the mineral interfaces : (i) the interaction with water and oxygen, resulting in the formation of the oxidation products, mainly hydrophilic; and (ii) the adsorption of xanthate, producing three types of products, on the surfaces of these three mineral samples, that form the hydrophobic coverage. On chalcopyrite, the adsorbed xanthate ions form initially a cuprous xanthate complex and a product with an absorbance band at about 1220 cm(-1). At higher than monolayer coverage, dixanthogen (X(2)) was found as an additional surface product. Dixanthogen is relatively unstable on the surface of the mineral after drying, and it was found only in in situ measurements. The assignment of the band at about 1220 cm(-1) is temporary, and it is proposed to be due to the surface iron complex, probably Fe(OH)X(2). In the cases of tetrahedrite and tennantite, the cuprous xanthate complex is observed in submonolayer and multilayer quantities. Dixanthogen was not found on these two minerals. Only on tetrahedrite was an additional band at 1227 cm(-1) observed at submonolayer coverage; this was assigned to a surface metal xanthate complex other than cuprous xanthate.