A fine-grained complex sulphide ore with high pyrite and pyrrhotite content was used to test for the influence of grinding media and pre-treatment with oxygen-deficient gases on the flotation selectivity between copper and iron sulphides. The tests followed a statistical design with two major levels of grinding environments, four levels of pre-conditioning, and three levels of flotation gas type. Additionally, an air-sparged mild steel mill was used to test the influence of the oxygen content in grinding on Cu/Fe sulphide separation. It is found that the grinding environment is the most dominant variable influencing the flotation results. Strong interactions exist between mild steel grinding, pre-conditioning, and the type of flotation gas. Test results shows that air-sparged mild steel grinding gives no advantage in Cu/Fe sulphide selectivity compared with ordinary mild steel grinding. The best selectivity is achieved after mild steel grinding and a combination of low-oxygen pre-conditioning and air flotation. Pulp chemistry data treated with multivariate statistical analysis show that the mild steel grinding, through some form of galvanic interaction, causes enhanced oxidation of the mineral surfaces, releasing more sulphur species into the pulp liquid. For mild steel grinding, the amount of oxygen in the flotation step and in pre-conditioning is most important, and the aeration during grinding plays a lesser role. Theoretical calculations suggest that the hydrophobic entity responsible for chalcopyrite flotation is a surface precipitate similar to the stoichiometric copper-collector compound. The dimer cannot influence the flotation results since it is not stable in the potential range investigated.