The depressant action of sulfite and metabisulfite (MBS) for copper-activated sphalerite has been studied in the absence and presence of sodium isopropyl xanthate (SIPX) under both air and nitrogen atmospheres by using the contact angle technique. Results obtained indicate that MBS is a better depressant of sphalerite in the presence and absence of SIPX under both air and nitrogen atmospheres. The simultaneous presence of dissolved oxygen and bisulfate presumably promotes their synergetic action on sphalerite depression. Results corroborate the beneficial effect of using an aeration stage before collector addition in order to maximize the depressant action over the hydrophobizing species present on the sphalerite surface and to avoid xanthate oxidation. (C) 2011 Elsevier B.V. All rights reserved.