Electroacoustic measurements (AcoustoSizer, Matec Applied Sciences) of zeta potential and particle size-distribution are reported on well characterised ultrafine (-5 mu m) sphalerite and galena particles. For both sulfide minerals the zeta potentials determined electroacoustically are in good agreement with those determined using the more conventional microelectrophoresis. The applicability of the electroacoustic technique to sulfide mineral particles is discussed in particular the influence of interparticle interactions, surface oxidation, pH and particle solids concentration.