The effect of iron, substituted in the mineral lattice, on the froth flotation of sphalerite has been investigated at alkaline pH. It has been found that a critical copper sulphate concentration exists where sphalerite recovery is maximized, above which the recovery of sphalerite then decreases. The presence of iron in sphalerite is detrimental to the rate of sphalerite flotation and hence its recovery, with the coarser particles being more affected that the fines. The presence of iron reduces the activation of sphalerite by copper, which in turn results in a reduction in collector adsorption. (c) 2005 Elsevier Ltd. All rights reserved.