Sodium sulfite was used to improve the separation of copper-activated sphalerite from pyrite in mildly alkaline pH conditions. This improved separation was mainly the result of the much larger depression of the flotation of pyrite than sphalerite. A further improvement in this separation was obtained by conditioning the minerals with oxygen and by decreasing the time interval between sulfite and xanthate additions. The results of spectroscopic studies have shown that sodium sulfite promotes the oxidation of copper on pyrite as copper hydroxide, but has no effect on sphalerite. This increase in the amount of oxidation products on the pyrite surface and, as a result, decrease in xanthate adsorption may explain the depression of the flotation of pyrite in the presence of sodium sulfite.