To investigate the possibility of using gases with varying oxygen activity and their influence on copper-zinc selectivity, flotation tests were done in a mechanical laboratory cell on a fine-grained complex sulphide ore with high pyrite and pyrrhotite content. The pulp environment was modified by changing the grinding media and by using different gases in the pre-conditioning and flotation steps. A statistical design with two levels of grinding media, four levels of pre-conditioning, and three levels of flotation gas type, was used. It is found that the grinding environment is the most dominant variable influencing the flotation results. Strong interactions exist between the pre-conditioning type and the flotation gas for mild steel grinding. Such interactions, and even the direct influence of pre-conditioning and flotation gas type, are statistically insignificant for stainless steel grinding, which often gave good copper flotation. The Cu/Zn selectivity shows a strong dependence on the pre-conditioning type and flotation gases for mild steel grinding, but not for stainless steel grinding. The best Cu/Zn selectivity happens for the combination of 5% oxygen pre-conditioning and air flotation after mild steel grinding. It seems that galvanic reactions after or during the mild steel grinding facilitate surface oxidation of sulphides. A mechanism similar to ferric leaching is proposed. As expected, it was observed that the oxidising or reducing environment in the pulp, as measured by the pulp potential, has a strong influence on the flotation results.