The mechanism of sulfur dioxide on collectorless and collector-induced flotation of chalcopyrite was studied by cyclic voltammetry on a rotating disc electrode, and by contact angle measurements. The first step in oxidation of chalcopyrite is the release of iron that upon hydrolysis may stay at the surface to render the surface hydrophilic. In stirred solutions, a significant amount of iron goes into solution to expose a copper-sulfur (Cu-S)-rich layer that leads to hydrophobicity. The Cu-S-rich layer reacts with the collector more readily than the original chalcopyrite or the iron-covered surface. In the absence of xanthate, the contact angles were larger when sulfur dioxide was present, indicating removal of some of the iron oxide at the surface. When sulfur dioxide and xanthate were added together, the contact angles were lower than those with xanthate alone. These results indicate that improved flotation may be obtained if treatment with sulfur dioxide precedes collector addition. (C) 2000 Elsevier Science B.V. All rights reserved.