The aim of this work was to find out a simple test reaction allowing the characterization of the acid-base properties of sulfided catalyst, This study was carried out using a silica-supported ruthenium sulfide phase as model catalyst and the condensation of methyl-thiol as test reaction, The choice of the RuS2 phase was dictated by its ability to lose an important amount of sulfur without changing its morphological and structural properties, The use of several techniques such as temperature-programmed reduction, high resolution electron microscopy, and X-ray diffraction have evidenced that about 45% of the initial sulfur content may be removed upon reduction while maintaining the pyrite structure, This reductive treatment modifies the concentration of coordinatively unsaturated sites and that of the surface sulfur species inducing by the way a change in the number of acid-base centers, The characterization of the interaction of CH3SH with these reduced states by Fourier transform infrared spectroscopy has demonstrated that this probe dissociates upon chemisorption leading to the formation of a proton and to a thiolate type moiety. This heterolytic character of the adsorption implies the presence of both a Lewis acidic type site to fix the organic fragment and the presence of a basic sulfur center to trap the positively charged hydrogen species, The combination of quantitative IR data with catalytic activity measurements has evidenced that the thioether formation is directly related to the concentration of thiolate species indicating that they are probably a reaction intermediate, It is thus concluded that the methyl mercaptan condensation reaction would give pertinent information on the acid-base properties of sulfide catalysts.