In a previous study, it was found that iron contamination from grinding media played a dominant role in depressing galena and chalcopyrite flotation (Y. Peng, S. Grano. D. Fornasiero, J. Ralston, Control of grinding conditions in the flotation of chalcopyrite and its separation from pyrite, Int. J. Miner. Process. 69 (2003) 87-100; Y. Peng, S. Grano, D. Fornasiero, J. Ralston, Control of grinding conditions in the flotation of galena and its separation from pyrite, Int. J. Miner. Process. 70 (2003) 67-82). In the current study, the effect of iron contamination on the flotation of galena and chalcopyrite on a particle size basis was investigated. It was observed that fine galena and chalcopyrite (-10 mu m) were affected by iron oxidation species emanating from grinding media to a much greater extent than intermediate size fractions (+10-53 mu m), and that grinding conditions had a more pronounced effect on fine galena flotation than fine chalcopyrite flotation. Mechanisms were investigated by rest potential measurement, ToF-SIMS analysis, mineral dissolution study and zeta potential measurement. It appears that fine particles (-10 mu m) are more easily oxidised than intermediate particles resulting in a higher concentration of metal oxidation species. Oxidation of galena and chalcopyrite enhances adsorption of iron oxidation species at pH 9 by electrostatic interaction. Fine oxidised galena has a stronger affinity for iron oxidation species than fine oxidised chalcopyrite. (C) 2010 Elsevier B.V. All rights reserved.