Surfaces of chalcopyrite CuFeS2, sphalerite ZnS, pyrite FeS2 and galena PbS modified by mechanical activation have been studied by means of infrared (IR) spectroscopy, diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) and X-ray photoelectron spectroscopy (XPS). The different oxidation species were identified on mechanically disordered surfaces of the minerals. The mechanochemical surface oxidation increases with the time of mechanical activation. The sulphides under study are partially covered with oxidised sulphates and carbonate species. They are distinguished by an affinity to the formation of surface compounds. According to the affinity of sulphides for mechanochemical S2- --> S6+ surface oxidation the decreasing rate was observed in the following order: FeS2 > PbS > CuFeS2 > ZnS. The ratio between bivalent and hexavalent form of sulphur depends on the degree of mechanochemical surface oxidation of the sample under study. (C) 2002 Elsevier Science B.V. All rights reserved.