Direct information at a molecular level about surface products, their structure and surface distribution is vitally important to perform an efficient separation processes for mineral beneficiation. This information is equally important for the modification of surface properties of industrial minerals for various specific applications such as: fillers, composites, weathering resistance, etc. The understanding of the mechanisms and kinetics of interaction of the first adsorbed organic molecules with mineral surfaces is the fundamental requirement to make possible the prediction, control and modification of the macroscopic surface properties that govern the efficiency of separation technologies or new material formulations. The developed infrared external reflection technique has a very unique ability to study interface phenomena at a molecular level on heterogeneous substrates. The variety, precision and reliability of information about interface phenomena delivered by this technique are superior to other single techniques. The experiments are fast and nondestructive. High sensitivity (starting from 20% of monolayer), in situ collected information in a multiphase system even in the region of a strong absorption of substrate, makes this technique a very valuable experimental tool. The complexity of the recorded reflection spectra, their sensitivity to any variations of the optical properties of all bulk and surface components and their spatial distribution in the system under investigation are in fact the major strength of the technique. In this paper, a few examples of application of this multidiagnostic technique for monitoring surface modifications of sulfide minerals in aqueous solutions will be discussed in detail. (C) 2003 Elsevier B.V. All rights reserved.