Gypsum supersaturated process water has been shown to have an adverse effect on the flotation of sphalerite minerals. This study probes the mechanism of such effect by determining the uptake of copper and Xanthate on sphalerite in gypsum supersaturated water using zeta potential distribution measurement, atomic absorption spectroscopy, UV-visible spectroscopy and quartz crystal microbalance with dissipation. Our results indicate that the supersaturation of water by gypsum retards the adsorption of copper and hence the sequence xanthate adsorption on sphalerite. The retardation of copper and xanthate adsorption on sphalerite is also observed by the addition of 800 ppm calcium. The adsorption of calcium is identified to compete with copper species for the reactive surface sites of the sphalerite, resulting in the reduction in copper and xanthate uptake and hence flotation recovery of sphalerite. (C) 2013 Elsevier Ltd. All rights reserved.