FTIR spectroelectrochemical measurements demonstrated that oxidation of galena starts with the reaction PbS+2xh(+) = Pb1-xS+xPb(2+), which is followed by the reaction PbS + 2h(+) = Pb2+ +S-0. Pb(OH)(2) is formed by the precipitation mechanism. Lead sulfite and thiosulfate and polythionate ions are formed from the elementary sulfur formed in the first oxidation stage. Dissolved oxygen participates in both the cathodic and anodic half-reactions for the galena oxidation. The anodic oxidation of pyrite occurs in two stages. At the first stage, the S deficient clusters or FeS defects degrade by reaction the FeS = Fe2+ +S2-, followed by hydrolysis and (electro)chemical oxidation-precipitation of the products (elemental sulfur, and iron hydroxide). The second stage starts with depopulation of the negatively charged iron acceptor surface states located at the valence band edge. After that, the bulk pyrite oxidation takes place through the thiosulfate pathway. It was shown that the sulfur formed on sulfides has a different origin: on pyrite, sulfur precipitates from solution, while on galena it is residual sulfur left after metal ion leaching from sulfide. The pyrite sulfur is oxidized mainly to polythionates and sulfate, as opposed to galena, for which the main oxysulfur compound is thiosulfate. The sequences of the reactions were explained based on the electronic band structures and correlated with the floatability of the sulfides. A new model of pyrite oxidation at the open circuit potential was suggested, based on the lateral electrochemical inhomogeneity of the pyrite surface. (C) 2003 Elsevier B.V. All rights reserved.