An experimental study has shown that the sphalerite/silica system can heterocoagulate despite both minerals having a negative zeta-potential [DiFeo et al,, 2001]. Using the model of Hogg et al., 1966 for interaction by constant potential, the interaction energy is shown to decrease between particles of like signs when the zeta-potential of one particle is between 0 and - 30 mV while for the other particle, the zeta-potential is < - 30 mV. These conditions include those giving heterocoagulation in the sphalerite/silica case. By comparison with experiment, a critical energy barrier is introduced which divides the system into dispersed and heterocoagulated states. (C) 2002 Elsevier Science B.V. All rights reserved.