The effect of copper sulphate and sodium isopropyl xanthate on the flotation of a mixed mineral system has been studied at pH 9. The mixed mineral system was composed of colloidal silica and ultrafine sphalerite particles. Batch flotation, zeta potential and rheology have been applied to identify mechanisms by which the silica may misreport to a sphalerite concentrate. The results indicate that, in the absence of both copper sulphate and xanthate, silica was recovered essentially by the entrainment mechanism. However, the addition of copper sulphate and isopropyl xanthate increased particle interactions between silica and sphalerite, promoting aggregation. Consequently, silica may misreport to the concentrate via a combination of both entrainment and aggregation mechanisms. Under the conditions applied in the present study, silica is not likely to be recovered via true flotation, i.e. by attachment of silica particles to bubbles. (c) 2007 Elsevier Ltd. All rights reserved.