Sphalerite from six sources is shown to aggregate at ca. pH 7-9, confirmed by different techniques (settling velocity, suspension analysis, and optical microscopy). This does not correlate with the isoelectric point, which is consistently < pH 6. A similar observation was made by Healy and Jellet [J. Colloid Interface Sci. 24 (1967) 41-46] for ZnO (and reconfirmed here), who suggested Zn hydrolysis products were responsible. The hypothesis is tested using chalcopyrite and silica suspensions in the presence of Zn ions. It is found that aggregation occurs over the same pH range, 7-9. Solution and surface analysis (extraction by EDTA) shows sphalerite released sufficient Zn ions to promote aggregation. The pH range 7-9 corresponds to hydroxide formation, suggesting this is the species responsible. Aggregation of sphalerite due to the polymerization/flocculation action of zinc hydroxide is proposed. (C) 2003 Elsevier Science (USA). All rights reserved.