The results of flotation of lead-activated sphalerite, in presence and absence of zinc sulfate, are presented in this paper. Zeta potential and adsorption measurements as well as thermodynamic calculations, substantiate that activation of sphalerite by lead nitrate is due to a replacement reaction: ZnS + Pb++ = PbS(surface) + Zn++ Depression of lead-activated sphalerite by zinc sulfate occurs by the reaction: PbS(surface) + Zn++ = ZnS + Pb++ The equilibrium constant for this reaction is modified to suit the conditions at the solution/mineral interface. (C) 2000 Elsevier Science B.V. All rights reserved.